You need to share that toy.
You should really do "x" for "y".
Can't you just do <fill in the blank> for me?
Have you ever just wanted to say 'no' - I can't? We live in a society where we are brought up from an early age to give and do for others (and I'm not belittling what a great trait that is to have). However at times, we need to think about ourselves. What is best for us in a given situation. What will keep our anxiety level from hitting new heights. I have always tried to do as much as possible for others (and I hope my friends reading this are not thinking they can no longer come to me with things - because that doesn't change). It is just that there are times when we cannot take on every project or allow that 'To Do' list to become so unmanageable that things begin to suffer. It may upset someone else to hear you say "I really can't do that now" - but in the end, your blood pressure may thank you for saying those words.
I am in the middle of several projects in my life - projects that many would consider selfish - but the balancing act is to have enough 'me time' along with spouse time/family time/work/etc. And there are some things that need to be removed from the list and put on the back burner. I recently decided to shift something brewing to that spot and turn the flame down to a low simmer and it made the knot in my neck disappear by just making that decision. And that was something I was placing on myself. Imagine if that thing in your life is something someone else is placing there instead!
Think of it this way. The person making a request of you is obviously having no problem asserting their selfish behavior when asking you. So perhaps it would be fine for you to show that same etiquette.
In the words of Nancy Reagan..."just say no." After all, it's not a four-letter word so your mother wouldn't mind.