We've been together for 9 years but I'm falling for someone else.

I’m really confused and could use some advise. I’ve been dating my girlfriend for 9 years and I do truly love her as a person just not as a partner anymore. I really don’t see myself marrying her in other words. Ive been going to this cute little cafe for about 6 months and I fell for this beautiful girl who works there. We talk all the time and just recently we’ve been hanging out. My girlfriend went on vacation and we hung out. She knows I’m in a long term relationship and I know she does have a thing for me as well. I never kissed her or showed any affection towards her because I am with someone else and I don’t believe in doing that behind someones back. Here’s where things get tricky though. I just signed a 13 month lease with my 9 year girlfriend and I don’t know how I’m going to make it. Please give me some advise on this mess I’m in.