I (21f) finally gave my childhood best friend (20m) a chance, but now I deeply regret it.

I fr need to vent bc I feel like the worst person in the world rn, and I honestly don't know how to handle this without completely breaking his heart. He and I have known each other since we were in school. He’s always been my childhood best friend. Looking back, the poor guy had been dropping hints for years flirty comments disguised as jokes but I literally just ignored all of it. To me, he was like a brother and that was all but over the last few months, the vibe between us totally shifted. We started getting way closer, the physical contact picked up, the conversations got deeper, and the flirting became real. Suddenly, I found myself looking at him differently. I thought, “You know what? He’s sweet, he knows me better than anyone, he treats me right... why not give it a shot?” So we recently started dating. The problem hit last weekend when things finally got intimate. I’m not even exaggerating... His dick was extremely small. I know how superficial that sounds, but I was completely in shock. I couldn't even hide my surprise in the moment, and to be 100% honest, I felt this awful mix of pity and turn-off/grossness that I just couldn't shake. Whatever sexual chemistry I thought we had evaporated in a split second. The worst part is that right after my initial shock, I panicked and just went through with it. I ended up having sex with him, but I completely faked enjoying it the entire time just so he wouldn't feel bad or notice how turned off I actually was. Pretending like that made the whole experience feel even more awkward and uncomfortable for me, and now I feel even more guilty for not being honest in the moment. Ever since that day, I can’t get it out of my head, and my physical attraction to him is like completely dead Now I feel stuck in an impossible situation. I feel like an awful, guilty person because he’s genuinely sweet, he cares about me so much, and it's not his fault. But at the same time, I can't force myself to be with someone when the physical attraction isn't there and the intimacy just isn't going to work for me. Like what's the point if it's going to be like just being friends I need to break up with him, but how do I end things without completely destroying his self-esteem? There is zero chance I'm ever telling him the real reason, knowing him, it would scar him for life and ruin the memory of our friendship. So I just need to figure out an excuse to break up with him without losing our friendship, and find our way back to how things were.