TMI but need to know

I recently had I guess like a chill and hook up kind of sex with a guy I’m pretty close to. We’ve known each other for years and we caught up on life, he confessed that he’s had real feelings for me for 3 years. I never knew until we started speaking more consistently about a month ago. I’m growing feelings for him. When we had sex, I found out his penis is quite large, and I’m small and it was pretty intimidatingly sexy to me. I felt like I could handle it. When we had sex, he was aggressive just like I was, and I love aggression. But when he would slide all the way inside, I felt a weird pain in my whole abdomen. I tried to moan off the pain but it just kept hurting when he kept trying to but I wanted him to go in deep because I knowww those deep strokes feel amazing. It did in the beginning but I started to freak out about the pain and we stopped, took a break and tried again. It still kept hurting when he tried to go deep again. Our solution was to do it missionary and he came twice on the towel. I wanna know why it hurt so bad for him to go deep? This has happened before when my ex tried to go all the way in too when we dated.