You’re not an asshole, not by a longshot. Dating is about getting to know one another and determine if you are compatible. It sounds like this isn’t working for you and for no other reason than you’re not feeling it. Which is totally normal and totally fine. This is a valid reason to break up with someone.
You will make her upset. There is no getting around it. But that doesn’t mean you’re a bad person. There is no magic phrase that you can utter that will not make her sad or angry.
What you can say are variations of the “it’s not you, it’s me”
“I was very flattered when you asked me out, but I don’t think I’m ready for this kind of relationship right now and I need to step back.”
“You are a great person and you deserve someone who can love you the way you deserved to be loved, but that isn’t me. I can’t give you the love you deserve.”
“You are a great person, but you’re not the right person for me. I can’t articulate it, I can’t force it and I won’t because one day I’ll resent you, and you’ll resent me. If we end this now, we can both heal and move on.”