Can't stop thinking about someone else

I'm a 23 year old guy, been with my girlfriend for 6 years now, living together for a little over a year. I wouldn't say we live a lavish lifestyle, living from payday to payday is starting to really get me down as my girlfriend only works part time she contributes very little financially, leaving me to deal with the rent/bills etc meaning I have little to no money to spend on myself. I work full time and any overtime I can manage to get, sometimes going in excess of 10 days working without a day off. This doesn't particularly phase me as I have a kind of fear about being in debt and will do anything to avoid it. Recently I have found it very difficult to be intimate with my girlfriend, I think this is attributed by (and this sounds horrible for me to say it) but my girlfriend has put on quite a bit of weight since we firsts met back in college and doesn't seem to care. As someone who is fairly active and physically fit this does upset me slightly. I have tried talking to her about it but it just degenerates into arguments and , frankly, I can't be bothered. At work I have known this other girl for sometime, but only within the last 18 months or so when, die to a reshuffle, started working with her in a much closer capacity. We get along really well, are both at the same stage in our career development, both work full time, we go running together, hoping to run next years London marathon together. I think I've always had a soft spot for her, feeling quite jealous when she introduced me to her boyfriend. Recently, they have split up and now all I can think about is her, I find myself day-dreaming about being with her, I get butterflies in my stomach when I am with her, when I'm with my girlfriend I can picture her being there instead. Just mentioning her name makes me feel like a teenager with a wild crush again. I really don't know what to do, do I stay with my girlfriend and carry on , or what? Any help is greatly appreciated. Thank you.

I think you have to decide if you really love your girlfriend. It can always be more fun with someone you don't spend so much time with and if its new. You said your girlfriend gained weight, I am curious as to how much weight she gained? Is it that you don't care about your girlfriend so much anymore or the fact that you are no longer as attracted to her? Attraction is very important so if you find your girlfriend more beautiful in general than this girl and genuinely care for her then why not go running with her or do more physical activities with her instead of this girl? I get she might get defensive but that probably comes from insecurity and I am sure she is much more aware of her weight gain then you are. She is probably just comfortable with you and feels you love her no matter what. If that is one of the main reasons then find a way to help you both eat healthy and workout. But if you feel that you will be happier with this other girl and she is truly worth leaving your relationship for then you should make that decision. There really insist much you can do about that but then don't be unfair to your girlfriend. She may know a guy who feels that way about her and she also deserves to be with someone who wants to be with her.