Is it too late for me?

Hello, I am 20 years old, and go to a public university in Pennsylvania and feel like ive already ruined my life. Im halfway through a biology degree with shit grades, probably below a 2.5, am not even interested in biology, and have no clue what I wanna do. The thing is, im very smart, social and positive as of today, but I was not so much this last coming year. I ended up getting a bit depressed, and that lead to a downward spiral off bad behaviors like excessive partying, drugs and other self defeating behaviors. I know I did not get a GPA to my full potiental, and know I could do much better. I feel stupid, like I wasted my time and life. Not to mention, I have worse ADHD then anyone ive ever met. I know this is no excuse, Im just saying this because I feel back on this excuse too much and didnt just make myself do the work that needs to be done. I wanma take it all back, get a perfect GPA, and show my potiental but I feel its too late, and im destined to by the average for the rest of my life. How could I get a 2.5 (or maybe lower) when I know i can do better? Is there anyway I can turn my life around and be the prestigous working man I want to be? I feel like halfway through college its too late to be the best I can. Hell I might even want to go to med school.. But not with a gpa like mine. How do I recover?