Gotta break it off with a co-worker and having MAJOR anxiety

[little mighty]

Short version: I have been "hanging out" with a male coworker (I'm female) for about a year, in some context or another. He's made it clear that he likes me as more than a friend, but truth is he annoys the shit outta me and I just want him out of my life completely. That's obviously not an option because we work together (same store, different department...but it's a pretty close environment), but i am having the WORST ANXIETY OF MY LIFE (and I'm pretty goddamn high-strung most of the time) to the point where it's affecting my physical health (weight gain, digestive problems, fatigue, brain fog...). I don't know how I can remedy this situation without hurting his feelings and/or making things awkward, but I know I can't live like this anymore.

Long version: I approached this guy last summer because I wanted to learn guitar, and several people had told me he was really good at it (he is). I was not interested in any more than that. We would meet about once a week, he'd mostly talk and I'd try to keep him on track (first indicator of annoying behaviour). It went from "okay thanks, see you at work" to "I'll walk you home" to giving me a giant hug. At that point, I knew he was interested in me and I didn't feel that same, so I made it clear that we were "just friends". (Usually this is a nice way of saying you don't want to see the person again, but he just WOULD NOT GO AWAY!! We kept hanging out because I didn't have the balls to end things at that point, and I also didn't have enough confidence in my feelings. He's the nicest guy EVER, and is very creative and fun, so I saw no logical reason to cut him off completely at that point.) He told me that he really liked me but he was fine with just being friends.

As time went on, thought, my feelings toward him started to change. We'd been doing stuff like watching movies, going on walks...basically hanging out every day*. Around New Year's, I asked if he'd still be open to a relationship, and he said yes. But as soon as the words came out of my mouth, I immediately felt sick to my stomach and knew I had made the wrong choice. We "dated" for about a month before I told him I was more comfortable as friends. Again, usually this is where the dump-ee gets the hint that you don't want to see them again...but not this guy! Stage 5 clinger of the WORST kind! This back-and-forth happened a few times over the next few months (dating, friends, dating, friends...) until finally shit hit the fan: our last conversation (two days ago) left things as friends. I felt a sense of lightness after that because he seemed to back off...until he texted me yesterday (like he does EVERY GODDAMN DAY) like nothing had changed and now I'm back to extreme anxiety.
*Nothing sexual had happened, and nothing ever did. Weird, I know, but it's true.

ALL I WANT IS FOR HIM TO LEAVE ME THE FUCK ALONE!!!!!!!!!!!!!!!!!!!!!!!!!!!! This rock in my stomach will not go away until he does. I'm not sure what to do because, like I said, I still have to see him at work (if he was just some random guy, I would've cut him off a LONG time ago and washed my hands of it).

I feel SO much guilt, shame, and anxiety because I feel like this is all my fault. I sent mixed messages because I was unable to be honest with myself, and therefore with him. All my friends kept telling me I was just scared of relationships (and they have reason to think that, long story short). But now that I have more confidence in my feelings as a result of what I've been through this past year with this guy, I am 100% sure that I have no interest whatsoever in seeing him in any context. But I'm afraid of telling him I need space because he hasn't done anything wrong, and I'll have to face the discomfort every day at work.

I really don't know what to do. I am overreacting as a result of my chronic anxiety and need to just not take things so seriously, or is this legit? Should I tell him I can't hang out with him anymore, or just ease up and keep him around because he's a good guy? HELP!!!! I just want to feel normal again!!!