I keep on doubting her even in small things

Just from such a short sentence, it's easy to see that you probably don't communicate well. What does communicate even mean? It means expressing thoughts and feelings in ways that are open to reciprocal expression from the other person. It's normal enough to doubt each other a little bit, but if it's constant, then you aren't talking with her in any meaningful way. It's also possible that you are both young, that you don't know each other very well yet, and that you aren't even really her boyfriend! How would we know? You have told us NOTHING.

So either help us out here, or go to a therapist. You might benefit from therapy anyway, if your parents were full of mistrust for each other, and yours was learned from them.

I keep on doubting her even in small things

WHY?!

Doubt comes from insecurity. If she has done nothing wrong then you need to look at yourself and work on yourself esteem. Carry on doubting and you may drive her away.

I keep on doubting her even in small thingsThen do her a huge favor... leave her so she can find someone who doesn't doubt her.

What would you do with "big things"?