It makes zero sense to worry about girls. You should worry only about how interesting is your life and how promissing is your life strategy. ANY girl may fall in love with some other guy anytime, including the case when she is still being considered as "your GF". You are not cutiest, richest, strongest or funniest man around there, so this can happen fo sure. You are OK, so just relax. Just be happy whilst you are together, but be ready to make some actions to find a new one when it become needed.
If she's not your GF yet then I don't understand what we are talking about. Make some attempts, if response is negative, just forget and don't even worry. Please realise that there are people who fit each other and people who don't and never will, and this is perfectly normal.