I really have a hard time with people telling Crusheddd on an anonymous 'net forum exactly what happened when they didn't see anything and do not know Crusheddd or his GF.
The GF is suffering from PTSD. That does something to a person. She said she did this so she could feel something after losing a baby. That's not blaming her A on the relationship.
Crusheddd,
I'm really sorry your GF cheated, and I'm sorry for your loss. I know life seems to suck at this point. I can say only that life will get better - more slowly than anyone likes, and it takes longer than anyone likes it to, but you can survive and thrive.
Your GF cheated because of her own issues, not because of any issues with your or with your relationship. You may feel like you screwed up, but you didn't.
You have choices to make. My advice is to figure out what you want and go for it if it's attainable.
As you see, some people will think you're weak if you choose to R. Others, however, will think you're strong if you choose R and weak if you don't.
Since you can't get universal approval no matter what you choose, Find the strength to go for what you want. It's your life, after all.
Note 1: Ordinarily I, too, would think ending a relationship with a pre-M/pre-kid affair is likely to be the best choice, but your situation isn't ordinary.
Note 2: Separate what you want from what you do. IOW, it's one thing to want R, but R is possible only if you are both willing to do the necessary work.
And that work is more individual than couples counseling. Your GF needs to look inside herself and change from betrayer to good partner.
You heal you.
She heals herself.
Together you rebuild your relationship.
Bu that's only if you want R. You can ed the relationship if that's what you want, and you can hold your head high.
*****
If you consider R, I recommend reading https://www.survivinginfidelity.com/healing_library/reconciliation/what-every-wayward-spouse-needs-to-know.asp?.
That has some information on how a good candidate for R behaves after d-day.
If it makes sense to you, I recommend printing it off, cutting off the URL lines, and asking your GF to read and maybe even discuss it.
I also recommend NOT telling her about SI. If she's not here you might be able to be more open about your thinking and feeling.