It's good to say yes, it's good to help people. Some people are jerks and don't 'deserve' help but it's still good to help them anyway.

Don't lose sight of the goodness!

Don't switch to being a 'No' person, they suck. Say 'yes' but say it with care. Next time that woman asks if you can take her out, don't say "I will let you know when I am free".

Instead check your diary straight away. Tell her "I can take you out for an hour at 4 pm on Saturday, I will take you to the park." Choose a time that suits you. Take her to a place where YOU will be happy and comfortable. After one hour, take her home, no ifs, no buts. If she doesn't like it, bad luck, she can take it or leave it.

Soon she will get used to your new way of saying 'yes'. And both of you will be better off for it.

This is my way to balance my desire to say 'yes' without getting burnt out. Hope it helps!