How to build trust again

My husband still communicates and helps out his ex with money. I think it is wrong for him to do that. There are no kids involved. He hides all the receipts of the things he has been sending to her. I confronted him about it and he asked me to trust him and that he has nothing to do with the girl. That's she's just a friend.

Is so hard for me to trust him. He has been doing this for a long time.