From my research and understanding there are only two unpardonable sins. The first is to knowingly attribute the work of our Savior to the devil. The 2nd would be to die without accepting Him as Lord.
Now, if you are having bad thoughts, here is something to consider. They very well may not be YOUR thoughts at all. You don't like them. They distress you. Therefore it seems apparent they are NOT your thoughts. And believe me the devil can put thoughts into your brain, and does that all the time around the world, but that doesn't make them YOUR thoughts.
As for demonic oppression I will give some suggestions. I hope they help! First, watch out for any unrighteous media - books, t.v. (which is most all of it), music, whatever. They often have ungoldly messages in them. Those kinds of things can attract evil spirits from what I have researched.
2nd, here is a form of spriitual warfare which has helped those in the Bible, and me personally. Singing praise songs! The Bible says that YHWH, aka God, "inhabits the praises of His people..." Where He is, the devil doesnt not want to be!
David gave Saul relief from evil spirits that way. Paul and Silas got out of prison, miraculously, that way. King Jehoshaphat saw enemy armies coming against Israel in overwhelming numbers. His priests went out before the enemy hoardes doing nothing but carrying the Ark (and the Ark, Yahushuah, aka Jesus, is in you) and sang praise songs. The result? The enemy armies turned on one another, killed every last one of themselves, and the Israelites had 3 days of booty to pick up from the field!
Once I worked in a place which I began to believe was filled with people who were into "the dark side." (There may be some such in your life, in the background, as they are getting more numerous all the time.) Sometimes at night I would have these strange visions in my head and other forms of what seemed like demonic oppression.
I prayed to know what to do. I was told to hum Christian songs. Singing is enen better, but I was in bed! This worked totally. I had to repeat it with future attacks, but eventually the attacks stopped.
Now, as someone trained as a speech/language pathologist I worry about things called vocal nodules, sometimes called singers' nodules. So, if you decide to try to praise singing, I would say to sing softly or you can get blisters on your vocal cords which will make your voice hoarse and be problematic generally. Soft singing works just as well as loud singing!
I would also say that if a song says "He is holy..." or "He is the light of the world..." or whatever that you can train yourself to be more directly in contact with the Almighty and sing "YOU are holy...YOU are the light of the world..." etc. If by any chance you have the gift of speaking in tongues btw, that works great too.
(The Bible says there is a kind of tongues that "no man understands" where you "speak unto the Father." Tongues are a wonderful gift of the Holy Spirit.)
Just some thoughts. I pray you will be led to do whatever is best for you personally.