Think I want to break up with GF, but too many ties...

I met a girl at the weekend & it really got me thinking about my relationship with my gf. I really fell for this girl and can't stop thinking about her. I didn't cheat, take her number and I'm not going to try to contact her. It's just cast a massive doubt over my relationship with my GF. If I'm truthful, I do want to split up. I just don't feel the same way about her as she does about me... We've been together for over 2 years, live together, have pets, she has no family in UK and has had suicidal tendencies in the past. I really don't know what to do. She's expressly told me it would break her heart if I cheated or broke up with her and I genuinely think she might try to commit suicide if I did. I definitely don't want to cause her pain but on the other hand - I have to do right by myself and my life. Anyone got any advice?

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