I'm with someone, but I have unresolved feelings for my ex-boyfriend

Every once and a while, I think about my ex-boyfriend from four years ago, and my current friend. It's like I can't get past it, and I need some kind of closure. I keep thinking about how we had something, but it got cut off due to parental intervention. Nothing was ever wrong with it. Now we've become friends, but there's this huge sexual tension between us, or at least I feel it when we're physically in the same place. Two summers ago, we saw each other casually for a while, but we never had sex then or while we were dating, which was only for two months three years ago. I'm now in my 20s, and my current boyfriend is amazing and in his 30s. Despite the age difference, I know we're a really good match. We've never really fought and are able to make compromises and talk everything out. I tell him everything. Also, my ex-boyfriend has a girlfriend now as well. He's been seeing her on and off for the past two and a half years, besides when they broke up and we had our casual thing. That summer, I told him I didn't want anything serious and broke it off. Only a week later, he had gotten back together with his girlfriend. His girlfriend is kind of mean spirited. I'm sure she may be different around him, but I worry about him committing to her in the long run. They're supposed to move in together soon. But still, I feel this connection with my ex-boyfriend, and I feel like he does too. I don't know what to do. I need some closure or I'm not going to be able to move on from this. I don't know what it is, but it's driving me crazy. I'm so happy with my current boyfriend, and I know he will do everything in his power to make my life beautiful and fulfilling. He's such a good person. I also know my ex-boyfriend smokes marijuana and does other things, but I can't get it out of my head. I'm trying to take a nap right now between classes, but I can't sleep because my mind is racing. I've never been so hung up on something for this long ever. I want to just talk it out with my ex-boyfriend, but I don't know if that's a good idea because of the possibility that it could blow up in my face. If he told his girlfriend, everything could go wrong because she hates me. If my current boyfriend knew, he would be crushed. On one hand, I want my ex-boyfriend to say no to having feelings for me, but I would be terrified of what to do if he said he has feelings too. Where would I go from there? On the other hand, I feel like I can't move forward in my relationship with my current boyfriend without closure from this. Sometimes I wish I could just cut off everything that had to do with my ex, but I don't want to. Please help me, because I really have no idea what to do.