Since October, Edinburgh have beaten Harlequins and Stade Francais, but dropped precious Pro12 points to perennial stragglers Zebre and Dragons.
"I think part of it has been that mental edge," said Dean.
"Harlequins are a big team, big game, and we followed it with a potentially lesser viewed team in Zebre, and they caught us on a bad day."
Moreover, Duncan Hodge's Edinburgh have produced poor first-half performances in their two most recent outings, consecutive European Challenge Cup clashes with Stade Francais.
In the first, they trailed 20-3 at the break - a game they managed to salvage in thrilling fashion - but 26-3 down at half-time in the return fixture in Paris last Thursday, they could not complete a similar comeback as they relinquished top spot in Pool Five.
Edinburgh host rivals Glasgow Warriors at Murrayfield on Boxing Day - a derby prospect Dean insists will help prevent any repeat of the first-half frailties.
"It's something we've been focusing on in training," said Dean. "And it'll definitely be a much-needed improvement for this game.
"Getting up for this game isn't going to be difficult and I think that's potentially where we were lacking in both those games against Stade.
"Credit to Stade, they really came out firing and caught us off guard, which is not ideal two weeks in a row, and it's definitely something we were pretty disappointed with.
"But that second half we came out firing, showed what we can do, and I do think they were there for the beating.
"This week, that mental edge is going to be there from both teams."
Glasgow are renowned for playing a high-tempo, offloading style and return to league action following two fine Champions Cup wins over French giants Racing 92.
The capital club are expanding the narrow, forwards-oriented game favoured by Hodge's predecessor, Alan Solomons, but Dean expects defence to decide the derby.
"When we're defending we'd like to slow them down, and when we're attacking we'd like to speed it up," he said.
"It's certainly a game that'll be won on defence. They're coming off the back of two good wins, but European rugby is a different type of rugby to the Pro12.
"They've got a few international boys back, they're raring to go and get one up on their Scotland team-mates."
Edinburgh centre Chris Dean says his side must be mentally tougher to halt their inconsistent run of form.