You can teach yourself the moves, and maybe develop some new tactics. But you really won't know how to fight unless you got into one. It doesn't matter whether it's in the ring, or outside. Experience and timing are the 2 keys to good fighting. Forget the rest.

Do the best with what you have at your disposal. I commend your efforts to quit smoking and do some more positive things. Since you have at least worked out with live partners in class in the past, you have some context that your solo drilling can fit into. However, you will also know from your experience that training without a partner can never truly prepare you for fighting as you don't have that feel for range, reactions and other things. Perhaps you can contact a few of your old classmates and see if they want to practice outside of class once in a while? I never also buy the "I don't have the funds" argument, as there is always a way to practice the arts if you make them a priority.

Take Krav Maga

Yeah. Buy a punching bag.

for fight everyone could, but you need good teacher to teach you right technique and everything about martial arts.

I'm an 18 year old male, right now I'm really stressed about everything and I also want to quit smoking and do something else to make me feel better, I've always found exercise to be a good way of doing that, haven't been doing much recently, but I had a punch bag installed in my room ages ago, and I was wondering if I could use that and teach myself how to fight, to do exercise and also something productive/different at the same time. I wouldn't have a partner to practice with, literally just this, is there any way I can accomplish it?

I should probably mention I did jujutsu for about 2 years as well before I quit, I don't have the funds to go back to that however, as much as I would like to.