Serious girlfriend (live together) but interested in another girl.

Ok, here goes. I've had a serious girlfriend for 2 years, and we've been living together for 7 months. We're 19 so we're young, but everything seems to go good. About 3 months ago, I met this girl when I went up to my friends college to go to a football game. Since then, we've hung out about 10 times (with other people, of course) and usually just happens when I go hang out with my friends at their college. She's my best friends sisters roommate so I pretty much see her when I go up there. Anyway, since about the second time we've hung out, I just think this girl is amazing. She's cool, funny, attractive, and just all around a great girl. Well, the other night I went up there to hang out and me, my best friend, his girlfriend, his sister, the girl, and a few others went out together. All I can say is we just get along great. We're always following each other, and when people split and start dancing with one another, me and her usually end up together. When we're walking back to the apartment or anything, we're always talking and end up separate from everyone else. Well, I know she likes me and I really like her. But she won't tell me because she knows I have a girlfriend and she respects it, which makes her even better. Like I was saying the other night, we really hit it off. Danced with each other all night, and it was passionate dancing...she started holding my hands and stuff like that, and I'm pretty sure she knows I like her especially after Friday night.

I''m just wondering what I should do. Is it normal to feel like "the grass is greener on the other side?" I mean our relationship is fine, but sometimes I feel like it's not the same as it used to be. But I think that will happen with every relationship...or most after living together for a while or whatever.

P.s. We also have stopped having as much sex...could that have something to do with it?

What do I do?

You are way too young to be in a long term committed relationship and you know what, here's a tip: wait til you are engaged or married before you live with someone. Find a dude to share a place instead...the transition after a break up is way better. Your GF is expecting a ring on her finger in a few years...have you ever thought about that? Dude you are not ready to settle down yet if you are mackin on this girl....it's very normal for a guy your age to want different experiences. Even if you say no to her, there will be others. This girl and other girls should be the thing you are experiencing in your life right now.....not a precursor to wedding vows and babies.

why would you be living with a GF at 19????! You are just setting yourselves up for the ultimate breakup! Now you are interested in another girl... guess what... the break up is gonna be MESSY!
Get out of this relationship now as you are too young and definitely not ready for anything serious. Move out. Go hump all the girls you can. Live life.

I understand where you're both coming from, and it's not that I won't break up with her or anything. I just don't want to.

I'm wondering if this is normal. I'm nervous because I want to be able to keep a relationship for a long time, whether it's now or in 10 years. I don't want to go through relationships and after a year and a half or 2 years, want something else. I'm just wondering if temptation happens with everyone in a relationship or even married, and you just have to be an adult and learn to not take it seriously.