There are two possible explanations for your situation. Either you are possessed by demons, or you only think you are possessed by demons. I'll give you some suggestions on both of these possibilities.
If you are indeed possessed by demons there is only one way to save yourself, and if my knowledge of horror movies has taught me anything, it has taught me that only an exorcism can rid you of a demon possession. You'll need a priest to perform this exorcism, he'll supply everything else needed.
The more probable answer to your problem is that you only think you're possessed by demons. There are many unexplained things in life, things that we tend to make up explanations for. You may have literally scared yourself into thinking you're possessed, you could even have split personalities and you consider your other personality to be a demon possessing you. Again there is only one thing to do, see a doctor, a physiatrist to be exact. Unfortunately you may not have the ability to see such a doctor where you live, if that is the case perhaps the exorcism will help you rid yourself of what you think are the demons. If you think a demon possesses you, an exorcism may help you think the demon is gone, thus making your life bearable again.
