Experience will help you to get over your insecurities... Seeing that people do keep their word... realizing that there are people out there with strong enough moral fiber to do the 'right thing,'... but you will need time to see this... time to learn to accept this as more than a possibility, but as real - something you could be capable of doing... and something that whoever you find yourself with are capable of doing.
At the moment you need to tell her... let her make her decisions on what to do on her end. Then take the time alone to realize how important happiness in life is to you... is it worth taking the chance and have faith that someone could be true to you? Is it worth the risk of being hurt... to be happy?
These things take time... it's all dependent on how long it takes to convince yourself that people can be trustworthy, that you too can be trustworthy... and more importantly you can find happiness in life.
"The weakest soul, knowing its own weakness, and believing this truth that strength can only be developed by effort and practice, will, thus believing, at once begin to exert itself, and, adding effort to effort, patience to patience, and strength to strength, will never cease to develop, and will at last grow divinely strong."
- James Allen