Why do you have these thoughts? Perhaps not the most relevant question. Perhaps a more useful question is "What would be worth focusing on?" Future rather than past focus.

Your thought patterns are like an addiction. You can use the same kinds of things an addict can use to get away from his habit. Try it. Worked for me. Do some research on this problem, obsess about it, solve it. There was a good book by Stanton Peele, called "7 tools to fight addiction." You can probably find it on the net as a pdf. Very useful advice. Reframe your problem as similar to an addiction and attack it directly. You'll have lots of otehr things to think about. If thoughts of heroin can go away, your intrusive thoughts can go away, too.

I only say this because you say you have no access to professional help. If you have to help yourself, I believe this is a viable approach. I am not a doctor or psych professional! Just someone who suffered intrusive thoughts at times also.