I totally understand where you're coming from. It used to be very difficult for me to read outside Horror and High Fantasy. The secret is to look for books that include elements of the genres you like but in other genres.

Like take horror for example - I always wanted to read Westerns but could never do it. So I found myself reading a couple of Westerns that had horror elements in them. I finished them and eased myself into the genre. After a while, I was reading more traditional Westerns.

Lucky for you, Thriller & Horror have so much crossover potential within other genres that you could easily find stuff out there to ease yourself in. My recommendation would be going in for some sci-fi since that's probably the easiest place to start and find horror and thriller elements what with scary space exploration or cyberpunk noirs and things like that. You'll be getting the genres you like in other genre settings, if that makes sense.

That's what worked for me, at least. Ultimately, the important thing is not to give up on branching out because once you're able to read and enjoy outside those starter genres, your reading is going to expand massively.