The past 3 times i've tried to make love my penis hasn't been able to get erect

I am currently 19 years old straight male and a sophomore at college.The past 3 times ive tried to had sex my penis hasn't been able to get erect. The first time was with an attractive girl in her bed and I was drunk, high, and on Ritalin as well (i know the side affects of Ritalin when it comes to the penis so i just shook that one off). The second time I was with a girl i would consider to be a very attractive girl. I had just smoked what could be considered a large amount of weed and we were outside in the bed of a truck. The third time I was a little drunk (not too drunk though) with a girl that was, to be honest, not that attractive. All three times there was foreplay and other actions that would turn the average straight 19 year old male on. But all 3 times i haven't been able to get erect at all.

I have only had sex with one other girl in my life. This girl I had been in a relationship for a little less than 2 years (end of sophomore year in highschool to the start of senior year of highschool). We had both lost our virginity during our junior year in very early January. We broke up for about 6 months, then became "friends with benefits" during our senior summer for about 4 months until we both went to college. We were pretty sexually active over the course of our relationship, and very very sexually active during the 4 months we saw each other over the summer. The original break up (back at the start of senior year of highschool) was a bad one for me, and to be honest i was heart broken and in a very low place for a few weeks.

My question is, when it comes to me not being able to get erect is it more of a physical problem or is it more of a mental problem? And what can i do to be able to get erect in future situations?