She's only said it to me I say I like her all the time my friend doesn't believe what I said help

If you have a girlfriend, unless this is grade school, you will be spending time together, you will be talking and walking to gether and doing things together.

Next you don't "PROVE" it to anyone, it is something between you and the girl. But if it is so hidden that someone that is close to you, can't see it, are you sure they are ?

Don't worry about convincing anyone of anything. Just enjoy your girlfriend while it lasts.

Don't get hanged up on what your friends think. Unless you want to objectify your girlfriend as a trophy, whether your girlfriend likes you or not is an issue between you two.

Good luck,
Javi