“I want the sweet kid who only thought nice things back.”
No one thinks only nice things. No one.
I think it’s also unrealistic to want to be the sweet kid you were when you were, well, actually a kid. You’re not a kid, so it’s unrealistic and unfair to yourself to expect that you could be the same as you were a couple decades ago.
If you haven’t spoken to a counselor or coach about this lately, I think it might be worth a shot. I would definitely recommend bringing this up with someone you really trust to respect you and what you share with them.
A therapist can help you figure out if there’s depression, anxiety, etc. that might be causing your thoughts to skew toward bleaker subjects. They can also help normalize some of your experiences and help you deal with the feelings you’re describing.
Having a “bad” thought isn’t the same as actually doing the bad thing. Having “bad” thoughts doesn’t make you a bad person.