Can’t get over the fact he had sex with someone else

I(24f) recently found out my partner(21m) slept with someone else after threatening me he would if I didn’t have sex with. I was giving him sex but I guess it apparently wasn’t enough. I do remember rejecting him sex because I was sick and he told me he was a big sex person and can’t rely on jerking it. So after he threatened to have sex with someone else I had sex with him. I got sick a week later from stomach flu and told him I couldn’t have sex.

He randomly joked that he had a std and said that’s the reason I’m sick. What I had my whole family had so it wasn’t a std. But since that day he was acting funny and constantly joking he had a sexually transmitted disease. After I started to feel better, he wouldn’t have sex with me even when I offered it, he was acting as if he didn’t care for it. So I got mad about him rejecting me and he made it up to me. We had sex like usual nothing unusual about him, no new sex moves or anything. Until shortly I get some burning urination and told him. He said I had just a uti and teased me about the infection the whole time. He didn’t want to have sex with me(at least unprotected sex) cause he thought I was going to infect him with my uti. He texted me the next day and asked did I still have “my infection” (probably wanting to have sex this day.) When I went to the doctor she told me it was a std. I got furious about and confronted him. He said he doesn’t have a std and I’m the one with the “infected kitty.” We got into a argument and I called him disgusting but he goes “you’re disgusting you’re the one with the std 😂, I don’t have it” It’s obvious he had sex with someone else because I’ve been 100% faithful. Why did he have the need to do that? I can’t think where I went wrong and how could he do that to me.

TL;dr: bf slept with another person and gave me a std. I have been 100% faithful. He says I’m the one with the std and he doesn’t have it. He threaten he would sleep with someone else if I wasn’t giving it to him which I got scared and had sex with him but he did it anyway.