How can i build up my immune system when im am feeling stressed, overwhelmed, depressed?

By taking care : You already know your problem(s) , take care of them with professional help , don't feel shy to see a psychiatrist , they are there to help you , you may even need medications , clear mind is the first step in curing any disease , including building up your immune system.

Exercise: Exercise may improve your immunity (including potentially lowering your cancer and cardiovascular disease risk) and hopefully your mental state as well.