Welcome to sf, Rose.
You're certainly not alone in suffering from thoughts like those that you have described. It's good that you've opened up about them here, but this place does not substitute for a professional. I recommend seeing your doctor to first determine if there is a general medical condition causing some of your symptoms, and also to tell him or her exactly what it is you're experiencing. Don't worry about thinking they won't understand. They've likely heard what you have to say many times in various forms.
Consider seeking out an empathic and caring therapist to help you with your intrusive thoughts of worthlessness, and of death and wanting to die. It can get tricky finding someone who is really helpful, but there are some out there that can help you cope with those thoughts, in the ultimate hope that they will go away completely.