How to refuse?

When one of your good friends ask you to help him do a thing which is a bit difficult for you,what can you do?You will refuse him or accept him?

Yesterday I come across this problem.I want to say sorry to him that I can't do it,but I have no courage to say so.Because I'm afraid I'll hurt him,so I receive his request.So feeble I am.

I'm the person who always do things like this.I don't want to make others disappointed,but in the end I am the person who is hurted seriously.

Because  I don't know how to refuse so I always hurt myself.I'm so puzzled that I don't know what to do and how to keep my friendship.

Who can tell me how I should do next???