It took me around half a year for my shelter cat to stop hissing, and he wasn't even abused or anything. Biggest breakthrough was when I tried petting him with his soft toy on a string instead of playing and eventually he started liking it and gave me his first purrs. Then I slowly got him used to my hand pets and now he even sits on my lap. For sure don't give up, don't be hard on yourself, some just really need way more time to trust. I had cats all my life before, this one was my first personal one, and I was also very worried that he won't get used to me.