The language of mortal sin reveals the urgency and gravity of the situation of sin but it can, unfortunately, also contribute to the problem mentioned above. It may lead some to think of themselves as too sinful for Christmas. But Christ is born in a space for animals and laid in a feeding trough. God comes to be with us IN our sin even as he frees us FROM our sin; such humility is his very nature and we must take that side of the equation seriously as well.
The following is typical of questions I have received over a number of years, but which come up more frequently during holiday times and seasons like Advent and Lent : [[Dear Sister, I am dealing with a recurring mortal sin. I have tried to develop a prayer life but then I go back to this sin and it prevents me from succeeding in that. I have had many people pray for me and have even had an exorcism prayed over me; I have fasted, used a discipline [a small scourge], but still I return to this sin. I am writing to ask if you will pray for me? Thank you.]]
I will certainly pray for you, but I need to suggest to you that you may be allowing the term "mortal sin" to prevent you from praying and developing a prayer life --- or from believing that you can do so. While it is good to have people pray "over you" or for you the simple fact is that the only real cure for our own sin is developing a relationship with God (and probably with others) despite our sin. At some point you need to be able to stand before God and trust God working in you and loving you in spite of your sin. In other words, continue to pray no matter what. God has not left you even if you turn from his life. He is always there knocking, waiting, offering his love (his very Self in act), and holding you in existence. Serious sin is certainly a significant concern but the term "mortal" can be misleading and cause us to believe God is not present to or within us, or that he ceases to love us while we are in such a state. Neither of these things is true --- not even in the state of "mortal sin." (Were they true, then repentance would be something we would have to come to on our own power.)
In Jesus' day certain illnesses were considered the result of demonic possession. When the disciples could not heal one of these, could not "cast out" the spirits involved, they asked Jesus why not. He answered that this kind of thing could only be done by prayer. The prayer was his own, but it was ALSO the prayer of the person who had turned to him (since turning to Jesus IS one dimension of prayer). Paul wrote that Jesus died for us (that is, God showed the absolute depths of his love in reaching down to us) "while we were yet sinners." Paul also was very clear about the conflict he found inside himself when he wrote, "The things I would not do I do and the things I would do I do not do." He then cried out in great anguish, "Who will save me from this body of death?" He uttered this as a man of sin AND prayer and his answer was a cry of gratitude to Christ and the God who worked to love him into wholeness in spite of (or even in light of) the great division in Paul's very self.
By the way, since you mention taking the discipline I want to say that personally I find that a typically masculine and frankly wrongheaded way to deal with some sins, but especially with sexual sins. (I do not know what your sin is, so forgive my assumptions here.) I once answered questions online with a number of other Catholic leaders. I found the priests there often counselled an approach resembling "going mano a mano with the sin" or "beating the sin (or our bodies) into submission." My own approach is very different. You see, unfortunately, with things like taking the discipline what is far more likely to happen is pain and pleasure become more and more closely associated in the person's mind (one's thoughts, images, fantasies) and brain (one's neurochemistry, neural pathways, etc) and one will begin to include or build sadistic or masochistic elements into one's compulsion. (I use this term because your own situation sounds like it has a strong aspect of compulsion.) This only complicates matters and makes things harder to deal with. It is far more helpful to be sure one's own life is full, balanced, marked by love (both given and received), and that it is "penitential" in more everyday ways which temper our appetites more generally while fulfilling true needs. (For instance, eating healthily, getting enough and regular sleep, taking time for exercise and other forms of recreation -- including reading, conversations with others, hobbies, etc --- can be considered penitential practices and have benefits which are wide-ranging.)
While this does not mean serious or grave sin is okay of course, nor that we can blow it off as nothing of real consequence, it does argue that you are a good deal more than this one sin and that you do NOT need to allow it to dominate your life. Of course, other things must do so instead ---- especially a life of prayer. Though I can make some suggestions here, in the main I would encourage you to find a good spiritual director who can help you in this; after all a prayer life is much more than a life of saying prayers. Not least, a good director can assist you in healing from the woundedness (including a woundedness sometimes of will) which may lead to behavior which is unworthy of being chosen. I would also suggest that you allow the focus in your own attention to move away from this sin (which is really a way of being focused on yourself) and onto the love and graciousness of God. Pay some attention to the ways in which you have to be thankful, the ways love is real in your life. Act out of that sense; celebrate the sacrament of penance out of that awareness rather than simply out of guilt and shame. In other words try to develop an attitude of gratitude and hope rather than one of shame, guilt, and even subtle despair. In my own experience real remorse with a true purpose of amendment stems from a sense of gratitude and the responsiveness it empowers more readily than it does from guilt or sorrow alone.