How do I back out of a potential relationship?

I'm currently in a situation where I thought I liked this guy but I really don't, at least not in the terms of a romantic relationship. Him and I used to talk a couple months ago but we stopped talking because we were just not ready for a relationship. We still remained friends and talk casually like normal. However, his feelings for me have been here since we stopped the whole thing and I thought that I liked him too so we slowly started talking again. Now I realised I don't really like him as much as I thought especially since I felt uncomfortable responding to physical touch or even just close distance. I already messed up but I really thought I liked but I just can't respond to him the same way he responds to me. Please help, I completely understand if I'm at fault.
How do I back out of a potential relationship?
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