Problem - How do I get ahead in life when so far behind?

18 years is not the end of life. You've got a lot of opportunities ahead of you. Just don't focus on what you've missed, but think about what you want to achieve. It's positive thinking that's the key to solving your problem. I know it's hard. A lot of regrets torment me every day - the wrong place to study, a lot of laziness, loss of time, low productivity, no girlfriend, or reliable friends. I understand that feeling. But I also understand that life has just begun. And holding on to regrets, you can drown in it.
I would advise you to start with mental health - deal with your stress, make a pleasant rest, find the motivation to move on, set a goal, and start to achieve it with all your efforts so that after time this goal also does not turn into another regret. This is the only way it will work.
And also take the time of self-isolation as a necessary measure for the future. Dedicate this time at home to yourself, spend it as productively as possible. Time to communicate with friends will be enough in the future, but there may not be enough time to take care of yourself.
Think positive! I hope you'll get better.

You must realize that you have not even started your life at 18. Therefore, don't be stressed out. Some people start their life at 40, some at 50, some even at 70, like Colonel Sanders, who started KFC. He was 70 years old when the idea of KFC came to him. Therefore, first of all, get rid of the feeling of being stressed, because the moment you are stressed, you cannot think, the mind can only think either a stressful thought or a meaningful thought. Use your intellect, and choose a path. Your life is just starting, you can become the happiest person in the world if you use your intellect to control your mind, to choose thoughts and this will determine the destiny of your life.