That joke, that witty remark held on the tip of your tongue; the cheerful smile for those who annoy you; that silence when you're unjustly accused; your friendly conversation with people whom you find boring and tactless; the daily effort to overlook one irritating detail or another in the persons who live with you ... this, with perseverance, is indeed solid interior mortification. (St. Escrivá, The Way)
That purification of the soul through interior mortification is not something merely negative. It is not just a matter of avoiding what borders on sin; quite the opposite, it consists of knowing how to deprive oneself, for love of God, of things that it would be quite licit to have.
This mortification, which tends to purify the mind of everything that is not God, aims in the first place at freeing the memory from recollections that would oppose the way that leads to heaven. Those recollections can assault us during our work or our rest, and even whilst we are praying. Without violence, but promptly, we will apply the means to get rid of them. We will struggle to make the effort which is necessary for our mind to fill itself once more with love, and a longing for the things of God.
Something similar can happen to the imagination. It can often upset us by inventing all kinds of novels, weaving fantastic fictions which are quite useless. Get rid of those useless thoughts which, at best, are but a waste of time. (St. Escrivá, The Way) Then, as well, we have to react quickly and return serenely to our ordinary task.
In any case, interior purification does not end with emptying the understanding of useless thoughts. It goes much further; the mortification of our potencies opens up to us the way to contemplative life, in whatever circumstances God has wanted to place us. With that interior silence towards everything that goes against God's wishes and is improper to his children, the soul finds itself well disposed for a continuous and intimate dialogue with Jesus Christ. In this dialogue, our imagination helps contemplation -- for example, when we contemplate the Gospel or the mysteries of the Holy Rosary. It is then that our memory recalls the wonders God has done for us, and his abundant goodness; and this will cause our hearts to burn with gratitude and ardent love.In Conversation with God: Advent and Christmastide
As queen of the "what if" scenarios I cannot tell you how helpful those comments by St. Escrivá about imagination have been to me over past years. Remembering to push away those "fantastic fictions" has saved me a lot of mental agony that has been totally imaginary.