He also continued to talk to that person (non-sexually on his end) for 3 months into our relationship because he felt bad for her (she put herself in a situation and he’s a good guy at heart and I guess felt like he had to try to be nice since he essentially dropped her for me).

So he had more respect towards some random fling than towards you, the girl he's just started a relationship with? Yeah, not cool.

Our first time, I was essentially manipulated into saying yes (I was afraid to say no and him dump me for not doing anything, as we were already a distance relationship. I know, ridiculous, and why would I stay with someone if I thought they’d leave over something like that). He confessed feelings he didn’t mean, while I genuinely reciprocated those feelings, though I feel it was a ploy to make me more interested in having sex. Ultimately, his side of the start of our relationship was manipulative and sketchy. But I stuck it through with the firm belief that that wasn’t who he really was.

Ehm, why are you with this guy?

I just desperately want to be able to fix myself

Why do you need to fix it? Some damage can simply not be undone. Furthermore, your bf is still occassionally lying about stuff and you find out.... so why do you actually wonder about why you're insecure? You're too young to waste your life in such an unhealthy relationship. Break it off and work on yourself. Don't let others treat you like a doormat and learn to communicate what you want.