So when did the trouble start for the original poster? Well, probably when she decided that it wasn't important for her to bother to maintain her marriage. 

So basically, she monkey branched out of her marriage into a relationship which, depending on where she lives will be a common law marriage at some point if it isn't already. Now she's worried that the same hypergamous strategy she used to get him will be used by the other woman. She apparently has no behavioral issues herself and in fact, she's working hard to change her partner into the man marriage material (cough, cough) if only this ghastly woman would stay away. It's interesting that she mentions not trusting is her partner, does this means that she assumes that the other woman is, in fact, trustworthy unless presented with booze and proximity to her partner. 

I recommend building a guest house on the property and engaging in polyamory, all the cool kids are doing it.