My Fe-dom friends seem capable of telling me when they are too tired, too over-extended, too busy to do something for me or with me. They have no problem with this, and I am so glad! I am the one who gets the big guilty feelings at times because I feel like I SHOULD always be helpful to others, even to the point of neglecting myself.
Thus said, I think the enneagram could be way more helpful here - it tries to pinpoint the emotional motivation behind our actions and might do a better job of understanding the situation rather than simply looking at MBTI alone.
A good quote to reflect on: "People with developed Fe tend to create implied bargains with other people about what sort of role they will play in regard to each other. They make an opening "bid" where they cast the other person in a positive role, e.g. "Would you be willing to spare a moment of your time to share your expertise with me?", casting the other person as an expert and a very important person. The implied contract is that if they treat you as a very important person, then you will each have very satisfying social roles. But if you get out of line, then you will withdraw your willingness to cast them in that role. Each line where you play the other person up thus creates a sort of debt: now they "owe" you some response that makes you important."