Me and my boyfriend have been dating for over a year now and I love him so much I really do but today I confessed to have smoked in September because I was so stressed and I felt so stupid for doing it. I just couldn't live with the guilt and this is the only thing I've done wrong in our relationship. And once I told him obviously he was upset and he told me its going to take a little bit for him to gain my trust back but we're still together thank god and he said he loves me no matter what. I hate hiding things from him I really do and that was the only thing but I told him. And now I'm trying to gain his trust back. How can I gain his trust back?

After a year, I would hope that you can talk about stuff.

"I hate hiding things from him"

So. Don't.

He does not trust you because you smoked and then told him about it?
Personally, I think he needs to get over himself.
Tell him you smoke when you are stressed. If he dumps you for that, I'd say you don't need him anyway.
Did you two have an issue about smoking previously?

No need to worry too much, just not to do things that hiding from him again.. as time goes by... automatically trust back will gain...

He should have asked what you were stressed about, but just be the good honest person you are and he will understand and trust you any way.

That is if he deserves and appreciates one such as yourself. If he doesn't to hell with proving anything to him, he will just have to get over it.

Trust should not be broken over this. At least I don't think it should.