I don't know whats the matter with me?

basically I've been dating this girl for nearly a year now and for the first few months it was great i couldnt get her off my mind.
but after a while i mind on random days had a tendency to wonder about splitting up and dating other girls and i have no idea why them ideas popped into mind head.
as the months went by i almost went to far and began to have a conversation where at the end of it i was gonna brake up with her.. just before doing so i ended up bursting into tears and regretting ever attempting to brake up with her.. since then i can't sleep properly and I don't know whats wrong with me.. can someone explains this?
I don't know whats the matter with me?
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