<human>: Give me the gist of the text in a nutshell

So I love my girlfriend of a year and a half, or at least I think I do. I'm the kinda guy who falls for girls really quickly, but the flip side is that it is really easy for the opposite to occur. The problem is that I just don't feel close to how I used to feel about her. My first relationship, year and eight months, ended my first day of college because I simply met a girl I enjoyed the company of exponentially more. I never got that girl, who I still have unrequited feelings for, instead I have had a few different relationships, including the one I am currently in.

Recently there have been major shake ups for me. Me and my girlfriend decided to try to have a threesome with a mutual friend (19/f) who it was obvious had a thing for me (even I knew and I'm oblivious) and I had started to develop feelings for her. We (me and friend) got along famously and even got mistaken for a couple when in public. My girlfriend has had insecurities about us hanging out because she even thinks we should go out sometimes. Lately my intimacy with my gf has dwindled, and I often fantasize about the other girl. I still love my girlfriend, and I don't want to hurt her, but I don't feel as happy anymore, and I'm beginning to hurt.
<bot>: Love my gf, relationship no longer making me happy, have feelings for another, don't want to hurt/betray her, don't know what to do. Help?