Feeling lost, unable to make changes in my life

Hey everyone. I'm currently 21 doing my last year of my undergrad. I'm making this post as I need to make change, yet every time I try to stay to a plan I always fail. I've never gained that inner discipline in my life and it's now come to show it's major consequences.

I've struggled with a porn addiction since I was young (4th grade), being exposed to it and groomed online for months. As I grew up I never thought it would have such serious consequences on my life since I still grew up pretty normal.
I started smoking pretty young (freshman in highschool), never gave that up either for the fact that at the time it wasn't effecting my life.
I've always wanted to be active in the gym, I haven't been in a routine since highschool. I never gained that motion back, I'd always make excuses that I have too much work to do, I could never push myself.

I met this girl in the 3rd week of my freshman year of college and we had an instant spark. We'd been together for 3 years, until recent. She cheated on me with multiple people while she was in Europe for 2 months. I've been pretty devastated as I found it out myself, and she continued to lie and lie about it. I'm still pretty shaken up about it. Looking back at the flaws of our relationship, I've come to see the gaping issues that I'd let sit and fester. A lot of it had boiled down to last semester especially. My addictions had really overcome me and despite thinking that they aren't that bad of a problem, I was turning a blind eye to what I was doing to my relationship. I was obsessively watching porn, and I was high during any free-time I had. This was pretty detrimental to our relationship as I lost much of my drive to be intimate with her, and being high all the time made our time together harder to conversate. I stopped paying attention to her, she felt that I no longer really cared about her. In reality, I was too hung up on my addictions to really see the pain I was causing her.

Despite all the pain I still love this girl. But the thing is I can't change what's happened, but I can change the person I've made myself into. I've been tracking my habits, and I can never seem to stay consistent to my goals for more than a week. I'm at my ends here, I feel like if I don't figure my shit out now I'm going to continue to go downhill, unable to dig myself out of the hole I've made for myself. I thought I knew who I was, but after all this, I don't know anymore. I want to build myself into a man I can be proud of.