I'm having a gay relationship on the side and am very conflicted

So this is going to be a long story but please bare with me. So I'm a normal, masculine 27 year old guy that has been straight for my whole life. Over the past few months I've noticed that I've started to have a strange attraction to one of my friends who I knew was gay. It started out with just texting more and hanging out more, but then it turned into hugging and sitting oddly close on the couch around Christmas time. A few weeks later my girlfriend went oversees for a few months for work and I started feeling really lonely. So this guy decided to come over and he stayed the night and we ended up kissing for quite a bit of time and sleeping in the same bed. The next few days we didn't talk about it at all and I felt extremely guilty. The next time we hung out I told him that it can never happen again, but before I went home we ended up making out in my car. I kept feeling really guilty about it but I could never make myself stop being around him. The making out turned into making out shirtless and soon completely naked and then oral sex. Not only was there a sexual connection but I also am in love with him and he is in love with me. I have never ever felt this way about a human being but I feel so guilty for lying to my girlfriend. I have cried so much because I know that what I'm doing is wrong but I can't stop myself. The problem is that I don't want to have to face the stigma of coming out and labeling myself when he is the only man I've ever had feelings for. I am so conflicted and I just don't know what to do.