user13144: I have an opponent that knows nothing about formal chess stuff, no glossary, no openings, and so on...
And yet that opponent destroys me every time with some DEEEEEP calculating, sometimes 20 minutes long for a single move in untimed chess...
I am nowhere good enough for that. I have ADHD and lose track of what I was calculating in my head mid-move, and every time this happens, I make some stupid blunder. (Also I tend to lose due to running out of time).
Is there any obvious play style I can start to use to go around that weakness?

user10233: Somewhat related to your question of playing styles, perhaps a good book on thinking process is really what you need. Organized chess thought is not obvious at times and takes practice. There are a bunch of good books out there written by GM's who give advice on how to organize your thinking during a game. Here are a few authors:
1: "How to Reassess Your Chess"  by Jeremy Silman
2: "Find the Right Plan" with Anatoly Karpov
3: "How to Choose a Chess Move" by Andrew Soltis
Of these three, the first is the easiest, the 2nd is the most practically applicable to play, and the 3rd is a little scattered. But, all three give you a good base for which to base decisions on. 
My favorite is Karpov's book because it is reproducible during games easily for me.

user-1: I have a rating of 2200, and I have similar difficulties with long calculations, but I have a good positional understanding.
My advice is:

Reduce your blunder rate by making a simple check to see whether you hang any pieces, walk into mate etc. Check out What is the most effective way of reducing blunders?
You don't need to calculate too far ahead. In my answer to How many steps can a grandmaster foresee?, I quote GM Soltis who argues that 2 1/2 moves ahead is fine for most positions
Play quiet and solid openings that don't have many forcing moves or tricky tactics. Try 1.d4, the slav-defense or the caro-kann.

This is a correspondence game I played on chess.com recently. My play was quite simple all told. I stuck to looking a few moves ahead and stuck to a plan of targeting white's isolated d-pawn:
[FEN ""]
[White ""]
[Black "Bad_Bishop"]

1. e4 c6 2. c4 d5 3. cxd5 cxd5 4. exd5 Nf6 5. Bb5+ Nbd7 6. Nc3 a6 7. Ba4? {White is out of opening theory now. Now I can focus on recovering my pawn.} b5 8. Bb3 Nb6 9. Qf3 Bb7 10. Nge2 Nbxd5 11. O-O Nxc3 {My plan from now on is to reduce material and target white's isolated d-pawn. This is quite a straightforward plan that doesn't need much calculation.} 12. Qxc3 Rc8 13. Qd4? {White should keep queens on the board, especially when my king is still in the middle. If queens were kept on, then I might have to deal with some tricky long variations.} Qxd4 14. Nxd4 Rd8 {Now I need to double rooks on the d-file to put pressure on the pawn.} 15. Nf5 e6 16. Ne3 Bc5 17. Rd1 Rd7 18. Bc2 Ke7 19. b3 Rhd8 20. d3 Bd4 21. Rb1 Nd5 {I saw an opportunity to trade minor pieces, seeing as the c3 square is weak.[%draw full,c3,red][%draw arrow,d5,c3,green]} 22. Nxd5+ Rxd5 23. Bb2 b4 {I wanted to add one more attacker onto d3, by playing ...b4, ...a5 and ...Ba6.} 24. Bxd4 Rxd4 25. Re1 a5 26. Rbd1 Ba6 27. Re3 R8d5 {I wanted to make sure that white could not start targeting my pawns with Re5.} 28. f3 f5 {If I could scare away the rook, then the d3 pawn is more likely to fall. Also, having my pawns on dark squares means white's light square bishop has no targets.} 29. Kf2 f4 30. Re4 e5 31. Rxd4 Rxd4 32. Ke2 h6 33. Kd2 g5 34. Re1 Ke6 35. Re4 Kd5 36. Ke2? {Because the d3 pawn is now pinned, I can trade rooks and invade with my king.} Rxe4+ 37. fxe4+ Kd4 38. Kd2 g4 39. Bd1? {White now cracks and loses the d3 pawn, and next the e4 pawn.} Bxd3 40. Bxg4 Bxe4  0-1


13144: <p>I have an opponent that knows nothing about formal chess stuff, no glossary, no openings, and so on...</p>

<p>And yet that opponent destroys me every time with some DEEEEEP calculating, sometimes 20 minutes long for a single move in untimed chess...</p>

<p>I am nowhere good enough for that. I have ADHD and lose track of what I was calculating in my head mid-move, and every time this happens, I make some stupid blunder. (Also I tend to lose due to running out of time).</p>

<p>Is there any obvious play style I can start to use to go around that weakness?</p>
 10233: <p>Somewhat related to your question of playing styles, perhaps a good book on thinking process is really what you need. Organized chess thought is not obvious at times and takes practice. There are a bunch of good books out there written by GM's who give advice on how to organize your thinking during a game. Here are a few authors:</p>

<p>1: "How to Reassess Your Chess"  by Jeremy Silman</p>

<p>2: "Find the Right Plan" with Anatoly Karpov</p>

<p>3: "How to Choose a Chess Move" by Andrew Soltis</p>

<p>Of these three, the first is the easiest, the 2nd is the most practically applicable to play, and the 3rd is a little scattered. But, all three give you a good base for which to base decisions on. </p>

<p>My favorite is Karpov's book because it is reproducible during games easily for me.</p>
 -1: <p>I have a rating of 2200, and I have similar difficulties with long calculations, but I have a good positional understanding.</p>

<p>My advice is:</p>

<ol>
<li>Reduce your blunder rate by making a simple check to see whether you hang any pieces, walk into mate etc. Check out <a href="https://chess.stackexchange.com/q/2596/1108">What is the most effective way of reducing blunders?</a></li>
<li>You don't need to calculate too far ahead. In my answer to <a href="https://chess.stackexchange.com/a/17089/1108">How many steps can a grandmaster foresee?</a>, I quote GM Soltis who argues that 2 1/2 moves ahead is fine for most positions</li>
<li>Play quiet and solid openings that don't have many forcing moves or tricky tactics. Try <a href="/questions/tagged/1.d4" class="post-tag" title="show questions tagged &#39;1.d4&#39;" rel="tag">1.d4</a>, the <a href="/questions/tagged/slav-defense" class="post-tag" title="show questions tagged &#39;slav-defense&#39;" rel="tag">slav-defense</a> or the <a href="/questions/tagged/caro-kann" class="post-tag" title="show questions tagged &#39;caro-kann&#39;" rel="tag">caro-kann</a>.</li>
</ol>

<p>This is a correspondence game I played on chess.com recently. My play was quite simple all told. I stuck to looking a few moves ahead and stuck to a plan of targeting white's isolated d-pawn:</p>

<pre><code>[FEN ""]
[White ""]
[Black "Bad_Bishop"]

1. e4 c6 2. c4 d5 3. cxd5 cxd5 4. exd5 Nf6 5. Bb5+ Nbd7 6. Nc3 a6 7. Ba4? {White is out of opening theory now. Now I can focus on recovering my pawn.} b5 8. Bb3 Nb6 9. Qf3 Bb7 10. Nge2 Nbxd5 11. O-O Nxc3 {My plan from now on is to reduce material and target white's isolated d-pawn. This is quite a straightforward plan that doesn't need much calculation.} 12. Qxc3 Rc8 13. Qd4? {White should keep queens on the board, especially when my king is still in the middle. If queens were kept on, then I might have to deal with some tricky long variations.} Qxd4 14. Nxd4 Rd8 {Now I need to double rooks on the d-file to put pressure on the pawn.} 15. Nf5 e6 16. Ne3 Bc5 17. Rd1 Rd7 18. Bc2 Ke7 19. b3 Rhd8 20. d3 Bd4 21. Rb1 Nd5 {I saw an opportunity to trade minor pieces, seeing as the c3 square is weak.[%draw full,c3,red][%draw arrow,d5,c3,green]} 22. Nxd5+ Rxd5 23. Bb2 b4 {I wanted to add one more attacker onto d3, by playing ...b4, ...a5 and ...Ba6.} 24. Bxd4 Rxd4 25. Re1 a5 26. Rbd1 Ba6 27. Re3 R8d5 {I wanted to make sure that white could not start targeting my pawns with Re5.} 28. f3 f5 {If I could scare away the rook, then the d3 pawn is more likely to fall. Also, having my pawns on dark squares means white's light square bishop has no targets.} 29. Kf2 f4 30. Re4 e5 31. Rxd4 Rxd4 32. Ke2 h6 33. Kd2 g5 34. Re1 Ke6 35. Re4 Kd5 36. Ke2? {Because the d3 pawn is now pinned, I can trade rooks and invade with my king.} Rxe4+ 37. fxe4+ Kd4 38. Kd2 g4 39. Bd1? {White now cracks and loses the d3 pawn, and next the e4 pawn.} Bxd3 40. Bxg4 Bxe4  0-1
</code></pre>
