Lies and broken heart

We broke up in January after a year together, got back together in April and promptly decided to get engaged bc we knew it was what we wanted. Well I just found out in early May while i was starting wedding plans he was texting another girl he used to sleep with and was flirting with her like crazy and trying to do things with her while we were together. To me, even though he didn't literally bone her (that I know of) its cheating. He swore that whole time "its in the past, she's just my best friend now and you have to get used to it" but now I know it wasn't.

I know he hasn't talked to her or the other girls he was friends with then since June, but we are now 4 months pregnant and getting married in 3 weeks and I just found this out 2 days ago and we fought about it and he feels so bad, but I haven't been able to sleep and I feel unwanted and unworthy and like a joke, a total fool. Can't stop crying, all I want is to feel like his only girl and I don't.

The struggle is he's said things back then such as "its only you" and "there's no other girl" and "I love you more than anything" which he says now but its lost its meaning since he lied when saying it the first time. I'm in a serious depression and extreme pain, sort of stunned if you will. I don't know how to forgive, how to see the truth that hes mine now and not live in fear of the past. Bc time is running out, but I'm just numb, empty, feel like nothing.