Why do I always think my girlfriend is cheating?

Two reasons: a) you don't trust your girlfriend b) you don't trust your girlfriend, and you haven't taken the time to talk to her about your feelings. Relationships are predicated and formed upon trust and open communication, so if you aren't taking the time to talk to your gf about how you are feeling, then she's never going to know how you feel and, more importantly, why you feel the way you feel. Talk to your gf. Let her know what is troubling you. If you have proof (which cannot be gleaned from this question - there are not enough details), show the proof. Do whatever you must to abdicate your feelings of what I can only assume is some nebulous cocktail of jealousy, contempt, and distrust. But, please, for your gf's sake, do not keep your feelings under a rug. For when the rug is removed and your feelings exposed, she'll see you for a lot more than you presume, and may ultimately leave you for not being open with her. Hope this helps :)

Why do you think your girlfriend is cheating? Is there anything that suggests that she is cheating on you? If there's no reason to think like that, then you're only making yourself feel like that and you shouldn't.

People often think that their significant other is cheating because their is an inherent lack of trust in the relationship. If you really want to make things work, talk to them about this distrust. It's worth the risk to get an answer once and for all.

This is usually because having codependency in the relationship. If she is away from you, you are unable to feel anything except neglected. Try to define yourself, do things you like and make yourself happy.

Because of the lack of trust you probably have with her. Assuming this may lead to a risky path in your relationship.

This may be due to your own insecurities or paranoia - I recommend you talking to her about these trust issues and find the root of the problem - it could very well be a problem from your end, a problem with her or something both parties must work on

You may feel like this due to the past? Trust issues or bad experiences- however if you love your girlfriend and she loves you you should be happy together

A lack of trust for others in your past maybe? Possibly even your present. Maybe she has given you a reason to not trust her? If that's not the case, maybe insecurities of your own are what's playing a part in this. There are definitely some kind of trust issues present in the relationship if you believe that she is cheating all the time.

Maybe you don't trust her or you don't trust each other, and this is really bad in a relationship. You have to figure this out and talk to her.

Perhaps you are worried or maybe insecure that you aren't good enough for her. Talk to her and communicate how you feel.