How do you tell someone to back off when they come on so strong?

Tell them how stressed out you are and tell them that you need to have support, not another person telling you what to do. Or you could say that you just need a break from everything and stay away from the person for awhile. Definitely explain to them how you feel before you take time apart though because if you don't explain yourself you could cause even more harm and stress for yourself and the other person.