"you know, I've never been here before. My #1 goal right now is to work on myself, and be the best version of myself I can be. I'm not quite ready to hang out, and I need to be true to myself and maintain my self respect"
waaaaaaaaaaaaaaaayyyy too wordy!
if you don't want to go with my gung-ho approach, I'd do this:
"thanks for the invite, but I am unable to do that right now". and then leave it.
Get rid of everything about self-respect, working on myself blah blah blah... she doesn't give a $hit.
Be as succinct and pithy as possible with being assertive and pleasant. Don't go into explanations about what you're doing and what not. They don't care.
