Zenfender I think you worry so much and can't be happy for long because you don't trust that you have control over your reactions to potential events in your life.
Life has its ups and downs, no doubt. It's up to you to learn how to deal with them effectively. Build mental toughness.
If you are young, chances are you won't be with the same girl for the rest of your life. So what? Enjoy the moment and appreciate the now. Carpe diem!