How do I get trust back slowly but surely in my girlfriend. I want her to start believing in me. I just need help

I think it depends on what you did. When my boyfriend cheated on me (im not saying that's what you did) I wasn't sure if I would ever trust him again. It is hard because he used to ask me 'what can I do to prove that you can trust me from now on' basically all I really wanted to know was that he was truly sorry and did regret what he had done.I had a lot of questions... why this and why that etc etc.. That if the trust was broken and your girlfriend is still with you even though you hurt her, then the only thing you need to worry about is making sure you don't screw up again and ensuring that when you apologised for what you did that it was sincere-look her in the eyes and say it from the heart, make sure she knows how much she means to you.the rest is up to her to believe you.