Since your question is tagged "setting-boundaries," I will tell a brief story about that subject. To avoid excessive use of pronouns, I will create two fictional characters, and give them names. Let's call them Jill and Mary. Are you sitting comfortably? Then let's begin.

Jill is a manipulator. She gets her way by emotionally manipulating others into giving her what she wants. She has been doing this for so long that she has forgotten how to work with other people in a way that is mutually beneficial. She pretends to have empathy, but only because it helps her get what she wants, whether that is love and affection, or merely the stroking of her ego. Jill may give the appearance of confidence, but in reality she is deeply insecure; she feeds off the energy of others to make herself feel better. In other words, she is a vampire.

Mary is a placator. Ask anyone who knows Mary, and they will tell you that she is a "nice" person. Mary likes to please others, and will go out of her way to make people happy. Because she is this way, she finds that she spends much of her time and energy doing things that meet the emotional needs of others, rather than her own emotional needs. Mary is empathetic, but that empathy comes with strings. Mary seeks validation by being validated by what others think of her. She may appear confident, but like Jill, she is also a vampire, just a more compassionate one.

What happens when Jill and Mary get together?

Mary cannot say no to Jill. Jill knows that she can get whatever she wants from Mary just by making her feel guilty. Mary already knows that she should say no to Jill. She knows that, by always saying yes to Jill, she is reinforcing Jill's bad behavior. Mary knows that she needs to grow a backbone, stand up for herself, and improve her self-confidence. But she also knows that, once she starts saying no to Jill, that she will catch hell for it. Jill will browbeat her mercilessly until Mary finally gives in, and the cycle will start all over again.

Nevertheless, one day Mary decides that she is finally through being co-dependent on Jill, and she tells Jill no for the first time. As expected, Jill retaliates, telling Mary how bad of a person she is and mercilessly trying to make Mary feel guilty about how terrible a person Mary is for not playing into Jill's games. A big argument ensues, and some breaking of glass is involved. But Mary stays calm, and stands firm. Eventually, Jill walks away from the argument.

Soon, Mary starts to notice that Jill is treating her differently. There is less anger from Jill, and a more conciliatory attitude. Jill begins treating Mary with greater respect. A few days later, Jill asks Mary to afternoon tea, and they have a nice talk. The argument is never mentioned. Eventually, Jill asks Mary if there is anything she can do for her.

Today Jill and Mary are great friends. They worked through their differences, and laugh now at how silly they used to act towards each other. But it never would have happened had Mary not decided to say no.

Like Stingray, I believe many good answers to this question can be found on the other post.

But I wanted to add that there is a book out there called, "He Is Just Not That Into You," by Greg Behrent. It was a cool book and was even made into a movie with Jennifer Anniston. Yes, the book is about drawing boundaries in dating and personal relationships, but these same boundaries should be placed in friendships, with co-workers and so forth, because they are true with any relationship. Despite it's funny name, the book is about respecting yourself, not feeding into others problems, not enabling others, and putting up healthy boundaries within all relationships. I highly recommend it, and if you read it you will also get a few laughs.

Mahatma Gandhi said, "A service rendered not in joy, is neither a service to the one being served nor the one serving." If I agree to something but I really don't want to be there, I want to be at home watching a movie I wanted or whatever, I feel resentful helping him. Now since I resent him asking for my help, he feels awkward and uneasy like maybe he shouldn't have asked for my help maybe he is being a burden on me? So we both feel guilty and bad about this situation, if I didn't help I would not have THIS GUILT but probably guilt over not helping him while I try and enjoy the movie I wanted to see so long.

The thing is to say no and forget about it, as it is not your responsibility to make him happy. This is the same for praying for someone, if I look at it as a chore I may as well not pray for that person as it wont be effective anyway since I have resentment that I have to do this, it must come from a real want to help the person.

Now if being used and abused that is different and clear boundaries are set easier that you can feel less guilty over or maybe even no guilt. However if you do feel guilt always remember you have rights and are worth more than being used as a slave for.