I know I'll get cal a **** for this, but anyway:
I've been with my boyfriend for years, longer than you, and as for you he was the first for me. We have always had a beautiful relationship, never cheated (or even thought of); we've been through some rough periods but nothing too worrying. I've never seriously thought of breaking up with him in all this time.
One day, years ago, I was attending an event that was gonna last for a few days. I didn't know anyone but quickly made friends during the first day, noone of them struck me in anyway (I remember commenting jokingly to myself that all the guys were pretty plain) To cut a long story short, by the 3rd day I felt my heart racing madly for one of them. I didn't even know how or when it happened - we got on so well, I felt amazing around him, it's not something I could help. By his behaviour I think he felt the same - maybe not as strongly as me, but you could tell there was an interest (also, these sort of things hardly happen completely onesided).
Have I considered leaving my bf and declaring him my feelings? Yes.
Did I do it? No.
I just sat there and told myself that I was still in love with my bf even if I had feelings for this other guy; I reminded myself that it's easy to be an amazing person for 3 days - a lot more difficult to be an amazing person (like my bf has been) for 3 years and more. I think feelings can't be controlled, but choices can; I decided to be with my bf (and I didn't do anything with the 2nd guy - not even spent any time alone with him) so I don't see why I should be blamed.
He lived a couple of hours away from me, so when the event we were attending was over I didn't take any chance to see him ever again. We texted in a friendly way from time to time for a few months, then he was a closed chapter.
It's been 2.5 years since and I believe I did the right thing staying with my boyfriend.