In the literature of saints there are plenty of stories of Christian men and women who allowed others to treat them abusively; or, less seriously, allowed even friends or loved ones to take serious advantage of them. I know there must be a difference between heroic virtue and a passive unwillingness to protect one’s own interests and/or dignity, but what is it and how can I practice this kind of heroic virtue without becoming bitter at being mistreated (even when you know intellectually that the mistreatment is petty and born out of inconsideration and not out of malice)?
St. Thomas Aquinas was once asked for the secret of how to become a saint. His two-word response was, “Will it.” That, I think, is the key to the distinction you are trying to make. A saint who remains silent in the face of inconsideration, mistreatment, hostility, and the like, consciously wills to do so in imitation of Christ and for the sake of the greater glory of God. A doormat remains silent because he is either unable or unwilling to stand up for himself.
When faced with the choice to stand up for yourself or to quietly allow another person to treat you badly, it may help to ask yourself which course of action offers an opportunity to do good. If the only damage done is to your pride, remaining silent may be a way to share in the Lord’s silence in the face of mistreatment (cf. John 19:7-9). But if you are becoming broken-down emotionally or are tempted to bitterness or have a responsibility to defend someone else, then standing up for yourself or someone else becomes necessary.