Yours is an important questions because, unless we make some important distinctions, you could wear yourself out, but it would not be because of forgiveness. First, let us discuss the issue of your offering forgiveness and the other rejecting it. Suppose, instead of the virtue of forgiveness, you were exercising the virtue of justice and every time you are fair to someone, he or she is not fair to you. Would this stop you from being fair? Woud you, for example, start to be unjust? No, you would persist. Why? Because it is good in and of itself to be just even when others are not. It is the same with forgiveness. It is good in and of itself to offer mercy even if everyone around us is unmerciful.
Now to the second point of wearing oneself out. You can practice forgiveness from a distance without necessarily reconciling with someone who continually takes advantage of you. In other words, forgive but then carefully consider what is fair and reasonable to effect a reconciliation with the person who could wear you out.