Worst relationship situation i've ever been in :( please help?

I had a good friend for a while that I hung out with often. I met all of his friends and became very close to them. This friend told me he really cared for me and wanted to take our relationship further. I had just gotten out of a three year relationship a few months before and I was very honest with him about not feeling ready to commit to someone new, plus I didn't know if I liked him in that way. But after much encouragement from his best friends and from a talk with him, I decided to give him a chance because I knew he was a good guy, and I thought maybe it's what I needed after the harsh breakup I went through. But after about two weeks, I broke up with him because I looked at him too much as a friend, and didn't feel the romantic chemistry he felt for me. I was also much closer to his best friend than I was with him, and felt much more comfortable talking to him and hanging out with him, so I was confused because I felt as if I was starting to develop feelings for his best friend. I couldn't really see it going any further with this guy, so instead of dragging the relationship on, I ended it. Apparently though, he had had feelings for me for about two years prior to this, so he was devastated that when he finally got to date me, it ended so quickly. I feel miserable that I hurt him so badly and I just want to be friends with him again. I completely regret making the decision to date him, and I know I should've trusted my instinct that dating him wasn't right for me. He won't even speak to me now. Here's the curveball...his best friend and I continued to hang out after the breakup. One night I drunkenly confessed to him that I had been crushing on him, and he was so happy because he felt the same. After that, we didn't act on anything ually (except for kissing), but it was literally just me and him hanging out together. We grew SO close, and have had the best conversations and talked about literally everything. We can act silly together, and have so much fun, and we can also get deep and personal together. I haven't felt a bond with someone like this in a long time, and we both are falling for each other so hard. We make each other extremely happy and I can actually let my guard down after being heartbroken so badly with him. I trust him completely. We're not really sure how this happened, it just DID, and it is so confusing and terrible because the whole time we were feeling so guilty and wrong knowing that we were literally sneaking around behind his best friend's back. This guy loves my ex and has been his best friend for 7 years. He was so upset because he feels so guilty, but we're both SO happy and we deserve to be. We eventually had and it was incredible, and nothing changed after, it literally just became better. We're falling for each other really hard, and it's wrong, but it feels amazing at the same time. But I am just terrified of this turning badly. If my ex finds out, he will probably freak out. As it is, he basically hates me for breaking his heart and has just been extremely pissy over it. If he found out his best friend and I have been sneaking around and actually have legitimate feelings for each other, their friendship will be ruined. I am so upset right now, because I don't want to keep doing this but I have such a good thing right now that I also don't want to let it go. I can really see myself being with this guy and having an amazing relationship eventually. We've considered telling my ex, because it would be less painful for him to find out straight up than by catching us in a lie, but we're both terrified to do it. Please help :(