If you’re having the thought that “I don’t trust my girlfriend,” it is highly likely that your relationship is in a serious and disturbing place. Relationships cannot function without trust (and not simply a base level of trust but a very high level of it), and the trust has to come from both parties. If thoughts of “I don’t trust my girlfriend” are going through your head, the first thing to do is to try and think about your relationship with as little emotion as possible. Are you imagining why you don’t trust her?
Has she given you any tangible reasons not to trust her, or are you simply operating on unfounded gut feelings (such as those based on her being attractive, her being away a lot, her liking to have time for herself, etc, aka things that could make it easier to behave in an untrustworthy manner, but that do not actually make it any more likely that she is being untrustworthy)? Sometimes, the most important part of the “I don’t trust my girlfriend” thought that comes to your mind is the word I; it can often be issues that you yourself have with trust or with forming connections with women that are holding you back and making you suspicious, more than anything she is actually doing (these can, of course, stem from past experience with other women).
If, however, she is giving you definite reasons not to trust her – being too flirtatious with other guys, hiding things from you, etc, and if you catch her lying to you, among other things – then you must ask yourself if the relationship is worth it to you to try and fight for (because you will have to fight to keep it going). If it is not, if she is not so special, if the main reason you are with her is because you’ve gotten used to the idea, or because you don’t like being single, or because you simply think she is physically attractive (among a host of other shallow reasons), you should seriously consider breaking up with her, or at least having a very serious talk with her about what is behind your “I don’t trust my girlfriend” thoughts, and threaten to break up with her if things don’t change soon.
If, however, you are truly in love with her, and cannot bear the thought of living without her, then you’re the idea that you don’t trust your girlfriend is terrifying indeed. First, once again, look at yourself. See if there is anything that you could do that would make her appreciate you more, and be less inclined to go behind your back with anything or lie to you about anything. Do you get too angry at her for little things (making her more likely to lie to you)? Are you too smothering? Do you make her want to get away from you?
Anything you can change about the way you are acting to make her less untrustworthy, you should. If, however, you feel you are doing all that you can, then you can either try talking to her, making her realize how much she is hurting you (because there is a chance she simply isn’t thinking about that consequence of her actions), and hope that she changes. If she does not though, you might be in for a heartbreak. A relationship without trust is surely doomed to fail…