I see a friend in a different/romantic way?

We've known each other for over a year - he's my neighbor. My roommate became my bf a long time ago...and my neighbor has ALWAYS been a best friend. He's always there when I need him and I tell him things I wouldn't tell others - including my bf. He has a gf but she doesn't know some things about him and prob wouldn't be okay (she's a GOOD girl). Anyway, we both really like each other and have talked about this - we're not cheaters! Nothing has ever happened! But we both want something to...we don't know what to do! We both can't wait to see each other, get nervous around each other...Gosh, I thought I was too old for this! Help!