This sounds lonely and sad. You didn’t mention when this started or how it is you are so alone. It’s difficult for me to comment beyond the very basics, but at least I can give you a place to start.
It’s never a good idea to assume that a problem is psychological. The place to start is with a medical doctor for a complete physical exam. You mention a decline in health as well as problems with memory. Even the thoughts that people are watching you can be due to a physical problem. Thyroid is sometimes a culprit. Vitamin deficiencies is another one. I’m not a medical doctor so I’ll leave it at that. Talk to someone who can do a competent exam.
If you come up fine physically, then it’s time to see a mental health counselor for an assessment and some advice. A counselor can help you figure out what to do about the negative thoughts and how to make friends. Cognitive Behavior Therapy (CBT) has been found to be especially helpful with negative thinking.
You made an important first step by writing to us here at PsychCentral. Now please take care of yourself by following up. You deserve to live a more healthy and happy life.
I wish you well.