To answer your question on whether anyone had been through similar and how you got through it...
I knew my (then) partner had lied about where he’d slept. I was told he had slept with someone, he flat out denied it, I decided to give him the benefit of the doubt.
Fast forward a few years later - I was contacted by someone on FB who apologised for sleeping with my partner. Confronted him, he denied it. There was a possibility that it was someone shit stirring for fun (she was a friend of someone who was known to do this). Again, he got the benefit of the doubt, I blocked her and moved on.
Last year, I found messages. He denied there was anything to it (see the pattern?) - I’d read the messages with my own eyes though and they were the messages of someone trying to get into someone’s pants. We split up briefly and reconciled after a health scare.
Less than 6 weeks later I found out he was cheating with one of my friends. Even with undeniable evidence he denied everything - I was a psychopath who wouldn’t allow him female friends, it was all in my head. He left me in a storm of righteous indignation that I could even think such a thing of him.
So, yes, I have ignored my gut feelings and allowed myself to be lied to. If confronted he will either deny everything or admit to something small like a kiss.
As for STIs - first thing I did was get myself checked out, I had inconclusive results - when confronted he lied about that too.
Sounds like the trust in gone in your relationship. When that happens it’s doomed. Sorry.