My husband has dated a LOT over the course of being poly for 25 or so years. I've only had either 1-3 dates with people, or long term relationships, so I count on him for wisdom in this stuff. His observations have been that if a relationship lasts past the 3 month hurdle, it is more likely to be a long term relationship. If by 3 months you find the person has changed their behavior as your boyfriend has, and you just realized he was not perfect, hopefully the NRE lifts enough for you to realize that this is not a relationship that should last.
I am glad you recognize it's not her fault, though you sure seem to have a lot of animosity for her. It'd be nice if it was your bf you wanted to push in front of the bus, maybe that'd make you realize you'd be better off if he got out of the picture. I think the truth is that for your relationship to be a success, it would probably take years of hard work, and most of that would be done by you, dragging him along, bashing your head against many stone walls along the way. I rarely give this advice to anybody but my advice would be - STOP DATING HIM. Maybe at some point in the future he will have matured, but I doubt it's going to happen anytime soon. Somebody who is a lying cheat at 3 months was never perfect, and obviously the communication you thought was wonderful was not. Save yourself some heartache.
Happiness will never come to those who fail to appreciate what they already have.