How do I tactfully reject a girl that I also like very much?

Long story short, a girl I really like and with whom I have spent occasional time (battling my fear and uneasiness around women) is giving me hints she may want something more - i. e. Keeping asking why I don't have a girlfriend, that I don't have to look far for one, etc. all while patting my shoulder or touching my arm.
The fact is, although I really like her and would want to try a relationship, that I'm not really in a good position regarding my anxiousness around women and I don't think I'm mature enough for a relationship.
So, how do I tactfully and gently make her understand (or outright tell her) that I don't deem it proper to move onto something more?
The real problem here is that probing around with common friends informed me she's a somewhat shy and introvert person, who's starting just now got move out of her comfort zone and engaging in social relations. Same friends also confirmed she really likes me that way, so I'm not overimagining things.
So, I'd really like to reject her in a gentle and tactful way, as to not undo any progress she may have done.
I'd also wish to make it clear the "no" should be permanent, so she wouldn't get any false hopes and be free to pursue someone else.
How do I tactfully reject a girl that I also like very much?
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