My girlfriend doubts me that I have relationship with other girl..

The only solution is to make her belive you and to make her trust you. Tell her that is too much gelous and you are only for her, tell her that she is the moast beautifull in the world, she is the only one who can make you happy and who can make your feature bright. Tell her that you live for her and you will not find anywhere a girl like she is and nobody will ever love like she does. Tell her that she give you a rare love witch is rare to find and try to make her to think you apreciate her love and it's that what are you doing actually. I hope it will help you!