Are you trustworthy?
The answer to that question is often the cause of the problem.
I would recommend that you:
1. Identify what it is you want.
[Example: to become a more trusting person]
Write it down. Yes, do it. It helps solidify your thinking. Refer to it often.
2. Evaluate YOUR OWN current behavior.
Write down the things you do and SAY right now that are NOT working or that you would deem as negative habits. Write down the things you do and say that are positive or that get you the responses you want from others. Refer to this list often.
3. Use this list of behaviors. Practice saying the things you want to say to others. Yes, set up a chair and talk as if the person you are trying to build trust with were actually sitting there. Yes, that requires work and some effort. How badly do you want to save or improve the relationship? Get off your ass then.
4. Plan ahead. Use your thinking to plan. If one of your strategies for changing the way you deal with people isn't working for you-Think! Come up with another plan to change your thinking and your behavior. Then apply that plan.
Lastly, stop "awefulizing" the situation. If she does something to break my trust I'll go crazy." No you won't. You'll learn and move on.
If you keep applying yourself, become a trustworthy person yourself, you'll find a trustworthy girlfriend. Become what you are looking for in others.
Hope that helped!