1.  If you are really jealous, you are (or will be) making both of you miserable.  If that is the case, the relationship will probably end anyway (unless both of you enjoy misery).

2.  If you have deemed her trustworthy, then trust her.  If she is not, then maybe it is best to let her go before you become more miserable and allow that misery to overflow onto her.

3.  If you aren't sure if she is trustworthy, then decide to trust her.  At some point, if she is not, it will become clear and you can decide what to do then.

4.  Find a way to make a decision and enjoy your life.  If you enjoy life and are a pleasure to be around, she will probably find herself wanting to stick closer to you.  On the other hand, it is hard to be around someone who is miserable all of the time.

5.  If the above doesn't work, you may have some unresolved issues from the past that need to be addressed before you can be at peace.

Hope this was helpful!

Without trust relationships tend to end and are not fun to be in.

Talk to her honestly about it and let her know how you feel.

In a relationship there are times you do need to work on things, compromise within reason and always communicate.

End of the day it is your feelings, your doubts and insecurities which you need to overcome and feel more confident about who you are.

Remember she has chosen to spend her precious time with you, you cannot be that bad right? =) So don't worry about others, just what you are to her. 

Early 20's you should be loving life, working hard and working out who you are not adding extra worries into your life.