Paranoia/anxiety disorder? PLEASE. HELP.?

I have a 'problem' I dwell on past mistakes (things I can't change.) I overthink everything, and make the worst scenario in my head. I basically need to learn how to settle down, and play my cards better. /: Also, I obsess over things, until I know its like 'perfect'. I will do things that I think will 'ease' my worrying, & I also, will start missing someone when they haven't even left yet.. I also have horrible thoughts of things, and people happening.. like somebody dying.. or something terrible happening to them. (loved ones) I don't know whats wrong with me, but I hate it.. So. Much. Please help.