As far as past sins that you had buried and are now bubbling up to haunt you, Christ's blood covers them. Just apologize earnestly to him, and repent (sounds like you've already turned away from them).
Recognize that the blasphemous and "unforgivable sin" thoughts are just an irrational function of your severe OCD. They do not represent your true will.
Have you been to see a psychiatrist? Their main function nowadays is to assess and diagnose problems, then prescribe medication that can really help.
I thank God that he has worked through doctors (and the first two were secular) to help me with medicine. Medicine has been a lifesaver for me when all the reasoning in the world had no effect.
I hope you will see a doctor who specializes in OCD--a psychiatrist. For genuine illness, it makes no difference if they're Christians or not. OCD is an real and serious illness. A psychiatrist might really be able to reduce your torment.
Proper medical treatment might really set you free.