Yeah, I agree with what others said: find interesting things you've always wanted to do and find people who maybe also like this too.. What have you always wanted to do or thought might be interesting?
If you have many interesting things and people in your life, just one won't be so important, hence less attachment..
Also, sometimes it can help to think of other people's flaws, so we understand they're not so perfect..
(My mum says about my Dad, 'Who's gonna want you?' lol)
What can also help: write BAD angry country songs about it or short stories or even a novel if you're up for it?
Prince Charles cheated on Diana with a definitely less beautiful girl, so looks are definitely NOT always a factor... It takes a lot of other things that are important in a relationship...
You don't wanna be too much of a doormat too, agreeing with everything and anything just so he wouldn't leave you is bad too.. (One guy said to me, 'If a gf agrees with everything I'm gonna cheat on her!' - huh?) He said there must be some disagreement so there's 'passion' (you probably don't wanna overdo it though, also guys' preferences can be different..)
Basically you wanna work on your confidence: what are you good at? What are your successes so far? etc. Maybe even have '5 things that went well today' or '5 things I did well' or such daily reminder..?
Oh, and stay away from canabbis. Some people have had very WEIRD experience with it, long-term usage can induce psychosis or other problems, some people can get psychologically/emotionally or
more addicted, some can go to harder drugs then.. (I've known some people personally who had problems with it.. Also check online, there's lots of info out there)