General advice:
People will always seek what is most advantageous to themselves. If you expect this as the default, you can be more proactive against the innate abuse.
It is not selfish, nor is it wrong to establish boundaries for other people when it comes to encroachment on your life.
It is not being a jerk, nor being an asshole to tell somebody that they are asking too much from you when they try to cross it.
It is not wrong or selfish to tell people straight up no.
Now, delivery is everything. While there is nothing wrong with any of those above things, you can make the assertion in a dickish way-- Try to avoid that. The important thing to remember is that people dont REALIZE they are asking too much; To them, their request/demand is perfectly reasonable 9 times out of 10. You have to be polite, and explain to them why it is not reasonable, THEN tell them 'no.'
Occasionally, people will have "made plans" that hinge on your agreement, and they will try to bully you. Remind them that it is very inconsiderate and downright abusive for them to plan your time for you, and that they should always ask before making any such plans, as that is the correct and polite thing to do. You can choose to give them a pass if this is your first time being assertive in such an interpersonal relationship, just assert that if they do it again, you WILL leave them hanging, and that to avoid that, they MUST ask your permission first. If they do it again after you have stood your ground like that, then you MUST leave them hanging, and be totally unsympathetic with their self-created plight. (They HAVE to learn that you are not their personal errand boy, sent by god to make their life easier. That means forcing them to experience the consequences of their abuse coming back to haunt them, and that means leaving them hanging. It is not being a dick. It is tough love.)