I am so very sorry. All I can say is this after being dumped with a 4 mth old baby & a 4 yr old...focus on staying in the moment....the actual present moment & your breath. Focus on what you have to be grateful for because no matter how much things suck, you have things to be grateful for. Get down to basics if you need to. Your child is healthy, you have a roof over your head, food in the frig, money in the bank, your GF, you are ok right now, right at this present moment. You don't have to figure this out. If you try you will drive yourself nuts. Sounds like you already know some things & others will become clear over time.
Your wife is infatuated with a new lover. Plain & simple. It has nothing to do with you not being good enough or her not loving you enough, she is missing something in her life & found it in this person temporarily. The chances that this is a lasting stable relationship remains to be seen. Maybe she will be back. I am sorry for you & I am sorry for your child. You don't just leave your spouse, you leave your child too.
Don't question poly b/c of one bad experience. Do you stop driving a car b/c you have 1 accident?
Me- Bi-female-early 40s- Single Mom to 2 young boys
Exploring & having fun for the time being after a long & unhappy marriage
There is no failure. Only feedback. -Robert Allen