It sounds like you are giving away too much of your own power/self to others, only letting her make you happy. You need to find your own reasons to be happy, in yourself, that don't involve other people making you happy (as they can't do that as they are not you; only you can make you happy).

It's very dangerous giving our control to other people as other people have their own lives to live and may/may not stay/leave, etc. Get to know yourself and your life doing your own things for you and you won't be so worried about her leaving you for someone else and will, in fact, be more attractive to her. You won't love her less or want to be with her less but will be more comfortable with yourself. If you have trouble thinking well of yourself, start by realizing she wouldn't be with you if you did not have attributes that are there for all other people to see; you should see them yourself.