I'm in a relationship but still in love with my ex. He's expressed feelings for me as well. Help?

I’ve been in a relationship with a good guy for two years and it’s serious. We even live together and have discussed taking the next steps sometime in the future. We’ve had our ups and downs and frankly there are some things he’s done that have upset me but he shows me genuine love and a desire to grown and develop as a person and within our relationship.

Here’s the problem: I still have feelings for an ex I split from four years ago. At the time, I was 20, he was 25 and it wasn’t going to work out because of where we were in our lives and distance (he was in the Airforce.) Let’s just say the feelings never really went away for either of us but we both tried to move on, see other people etc.

He’s confessed to a friend of mine that he’s still harboring romantic feelings for me and that I was the only woman who’s ever made him happy. I feel the same way for him. I care so much about him, it’s unlike anything I’ve ever felt. He understands me, makes me feel safe, doesn’t judge me or nitpick, lets me have my freedom. We just “get” each other and I’ve never enjoyed anybody’s company so much in my life.

Last week, I spoke to him on the phone for nearly three hours after several months of non-contact (without my partner’s knowledge, he would be so upset.) and it felt so right and so comfortable that it’s been on my mind since.

I want to see him again. What do I do? Do I throw away a two year relationship I’ve worked hard on, after we’ve been through so much together? I don’t even know if I should be in a serious relationship right now at all, but I really enjoy this person’s company and would like to have him in my life again in some capacity. But by doing so, I’d have to leave my current partner.

More torn than ever…