Dating Advice: Who do I choose?

I come to the OT for some advice on my current dating dilemma.

I've been seeing this girl for about six months now and things are about as serious as it can be in a relationship where neither party has professed their love for the other individual.

She is the exact opposite of me in most respects but for some reason I find her differences to be quite appealing. She has opened my eyes to many new interests and beliefs and has actually helped make me a more well-rounded person. I get along great with her friends and my friends seem to like her friends as well (my best friend is even starting to talk to her good friend). The sex is outstanding and I can tell that she really cares about me. Everything has been fine...until a couple of weeks ago.

This really cute girl came into work a few times and I wound up chatting with her for several minutes each time. The other day she and I were actually able to talk for nearly an hour and I discovered that she has a thing for me. We exchanged numbers and then she texted me a couple of days later and we continued communicating throughout the day.

I began to feel guilty about openly flirting with this girl so I admitted that I had a girlfriend and told her what was up. I told her that I honestly find her very attractive and interesting and I would actually like to get to know her just as a friend. Although she was disappointed about my revelation, she surprisingly agreed to my proposal and we met each other a few days later.

We walked and talked for a couple of hours and I was able to get a better glimpse of her personality and how we interact with each other. We parted ways at the end of the night and she texted me later that she had a great time and wanted to do something again.

We met again the other night and had a great time with each other. Hell, I consider it to be one of the most enjoyable evenings I have ever spent with a female (that didn't end in sex).

She told me her friend thought it was weird that I wanted to hang out with her despite having a girlfriend. I replied that I thought it was perfectly fine and asked her how she felt about it. She said she also thought it was a little weird but that she never said that her intentions were pure.

I don't get the impression that she is just looking to hook up. She is actually pretty conservative and soft-spoken.

I really like this girl and consider her to be more of what I've always looked for in the opposite sex. However, she is a few years younger than me and still lives at home so I don't know if there could be some potential maturity conflicts at some point down the road.

My girlfriend and I get along great, but I have always felt that something is missing in our relationship when it comes to physical attraction.

Have any of you dealt with a similar situation before in your dating life?