Hey, just wanted to say, as someone who's been involved with an m/f couple for the last two years (only dating one of the two, but close to them both) -- insecurity is very, very natural. There have been times when I've actually been jealous of them for spending their anniversary alone together (!)... what sense does that make?!? Well, it doesn't make any sense, but that's the thing about insecurity and jealousy, they're based on fear and not necessarily reality.
Don't get me wrong, most of the time it's been awesome.
But I definitely have struggled with worries, big and small, all along the way, related to being the new person, the third person, the less serious person, etc etc. I think it's just a position that's naturally set up to make you doubt yourself or feel out of place at times. If you can be strong enough to analyze your thoughts and feelings, to figure out what's grounded in reality and what's not, if you can handle some uncomfortable feelings without freaking out, it gets easier with time. And it can be totally totally worth it with the right people!
Best of luck!!!