Dating question: What's the best way to politely say no?

A guy recently asked for my number, and I didn’t really think about it and just gave it to him. If I were single I’d definitely go out with the guy, but my current dating situation is messy and terrible and I cannot include anyone else in my mess. Even though technically I could go out with him if I chose to.

He sent me a friendly text asking how things are going. The easy way out would be to just not respond, but that seems immature. Do I keep up the text correspondence and politely decline when the inevitable “let’s have dinner” text comes? Can I tell him, it’s not you, it’s me? Do I try to explain why, or just say no thanks?

Anecdotes welcome.