Welcome to MSF.

I'm sorry you're going through this.

Read your post back and I think you will find your answer: "he will not acknowledge how unfair this is all to me, he will not reassure me, or even apologize..." <THIS

He's not a good boyfriend, even if she weren't in the picture. You've only invested 4 months (thank goodness!). So get out now and don't lose anymore time with this guy.

Also, check out Natalie Lue's Baggage Reclaim blog/podcast. It's very good.