Letting him down nicely­čśö I'm bad at saying NO

i went on a date last week with a guy from my school and it was okay. we had dinner and walked around and talked for awhile. I guess i fucked up by going out with him in the first place, because i knew that i didn't like him like that. but i recently got out of a heartbreaking relationship and i thought going on a date with someone else would make me feel better, but it did the exact opposite. I thought about my ex the whole time and ended up hanging out and sleeping with my ex immediately after the date with the other guy.

now this new guy won't stop texting me and he's always randomly showing up wherever I'm at at school (we're in college) and i know that i put myself in this dilemma but i just don't know how to tell him i don't wanna go out with him again without hurting his feelings. i just thought i was ready to see other people but I'm not, not at all.

what are some things i can say to this new guy to let him know I'm not interested without it sounding as bad as it is?