i'm so sorry
Pranava - I really feel for you.
I have actually been in both positions - thankfully in my mis-spent youth, although I am totally in sympathy with you and your hurt feelings, your long term relationship and your coming baby.
What your DP is feeling is like a fever, it is all consuming and unfortunately probably nothing you do or say will get through to her. It's like she is on another planet - and all she wants is this other exciting young thing. It's really sad. And completely hopeless. In my experience the one who "loves" two gets neither and ends up miserable.
I know that probably doesn 't help you now.
I have also been many times on the receiving end of a loved GF being infatuated by someone else and having to try to "win" her back. It is futile. And so painful.
I know you have probably heard this before, but my advice to you is similar to what others have posted, and it comes with love and understanding, is to focus on your plans for you and the baby. Try your best to make plans so that you are able to live by yourself or with someone else, support yourself or get help from others like family and friends. Do not wait around for her to come to her senses. You have you and the baby to consider and she is too busy with her fling to even care about that.
I agree that the therapy could help you both at least sort out some details, make a break that is as clean as possible and deal with some of the feelings.
Do you have any leverage to get her to go to therapy?
Also you might want to think about joining a support group IRL as well as on MBs, getting your own personal therapist if you don't have one.
Know that you can and will survive without this person. Even if it is so painful and you really want to make it work. I think if she is willing to make it work that is one thing, but it sounds like she is not interested at all.
My heart is heavy for you. Remember you are good, strong, beautiful person and you will make a wonderful mother.
I hope I wasn't too blunt. I just recognize the story so well that I have strong feelings about it. I did not like myself one bit when I was the one that strayed. I had to really do lots of growing to get out of that pattern.
Yours, a very happily reformed gigoletto!
PS if she does come back to you and is interested in a few months time, you will be in a stronger more independent place from which to weigh your options.