I have changed

I'm 19 and I have changed so much over this past year... it drove me and my boyfriend apart and I ended up breaking up with him because he didn't like the "new" me. I drink a lot, I smoke cigarettes and weed (drinking, smoking and weed is legal in my country though), when I was with my ex still, I cheated on him even though I love(d) him more than anything. Why is this happening? I don't want this. These are all things I said I would never do. I don't want to be this person but it seems out of my control. I feel like a failure. Does anyone have any advice?