I recently had an a ha moment.
For years, I've been dating non-poly women thinking that eventually I'd happen upon one that is poly. And, somewhat dismissively, I've held for a long time that everyone is poly if they could just strip away the layers of culture that bind us all. So, I thought I'd get lucky, so to speak, and meet a women who otherwise didn't identify as poly and we'd make it work.
My a ha moment came when I realized how unethical this was of me. I was dating women who identified as monogamous (well, all women I meet, generally, identify as monogamous). To be ethical, I must respect their stated desires. And, I had to be upfront with them about who I was and what I able to provide in a relationship. The truth of this has essentially caused me to stop dating mono women altogether.
The OP is not poly. Any advice that asks her to work it out or give it time or space seems misguided to me. She's telling us, whether she's said so directly or not, that she longs for a monogamous relationship. I don't really need to know anything else about her boyfriend or his wife or their kids to reach what seems to me an obvious solution.
If OP doesn't want poly…then she'll never make this work. And, assuming that the boyfriend DOES want poly, it's unethical of OP to remain.
Poly shouldn't be something you endure. I, for example, couldn't endure a homosexual marriage; not because I might not care for the man I was with, but because homosexuality is not in my nature. Poly is a nature that you either have or don't have.