Perhaps instead of "yes" or "no", you can start saying , "Let me get back to you after I check my schedule. Things are kind of hectic now and I don't want to commit to anything I can't do." Then wait a while . Do an honest evaluation of
your needs and
your time. Sometimes you may still be able to say "yes" but ii it's "no", you can beg off and tell them you just can't work it in.
So many of us are conditioned to just say "yes" without thinking. This method let's you still be honest and giving when possible. Me, I've gotten to the point where I just say 'no", but then I'm way older than you and probably not nearly as nice.

(Although I did ease it into it.)