Low Vitamin D, magnesium, Vitamin B12 can cause anxiety and depression
Blood tests can see if you're deficient.
These are common symptoms of anxiety and panic disorders, you should not be worried. It can easily treated by medicines and regular therapy. You should start counselling and go for proper treatment.
One symptom of panic/anxiety is a "feeling of impending doom." So is being afraid of death. I've experienced both. So you can safely assume with confidence that this is what's going on, and nothing more.
It sounds like you're dealing with agorophobia which basically means you get frightened when away from home and would prefer to stay there as much as possible. I encourage you to get counseling because 21 is too young to be a homebody!
I wish you well.