Meeting the one ?

How do you know you met 'the one'?
I was almost certain I had a year ago in a man whom I was dating a second time. He's the perfect guy for me aside from having some issues I dont know if he'll ever do anything about even though he wants to. He said it was love at first sight in a way hes never experienced and he still sends me happy birthday and such but we decided that we are probably imagining that we are the best match for each other... He's seeing a chick long distance since then and i'm talking to other guys. Trying to find the one.

So i'm feeling pretty bummed. Everyone around me found love very young, true love. I've never had true love, i've thought i've had but I haven't. Maybe it's because i'm an avoidant person who don't share anything with anyone but I try. I'm not even sure I can fall in love anymore. It's strange after the last guy I have felt hopeless and done with love in a way i've never have even after bigger letdowns than this, guys I had known for longer periods of time and such. I've always been able to pull myself up. I don't know what is wrong with me now. Maybe I still have some tiny chance though, I was convinced to go back on dating sites so when I start dating again.... I was wondering how to tell someone is 'the one'....If I havent already met that person before and lost them again.... Life is funny sometimes so.