[Feel free to submit a question for this column, but please consider that it has become very popular and there now is a backlog, it may take me some time to get to yours.]
A. wrote: I could really use your advice on something that has been bothering me lately. I have been trying to practice Stoicism for quite some time and it has really helped me in several aspects of my everyday life. Unfortunately, a month ago my mother was diagnosed with an extremely rare and rather aggressive type of cancer. If she had not been at Stage I she would have only a few months of life. Still, the prognosis stays poor, but there is some hope. To cope with the situation I go through over and over the basic principles of Stoicism about death and grief and that has eased my pain a lot. I am focusing on the time I have with her trying to make the best of it, feeling blessed that she is still in my life without neglecting my other duties as a wife and a mother. What really confuses me right know is this. In order to prepare myself to get familiar with the worst case scenario, I have started to implement into my daily meditation routine the practice of “premeditatio malorum,” where I imagine her last days, the suffering, and her passing away.
Although I fully understand that this exercise is extremely helpful, mostly in terms of finding courage to deal with the upcoming difficulties, it also leaves me every time extremely overwhelmed, miserable, and spending time worrying about something that is out of my control. At some level, I think I’mm being ruled by fears triggered by excessive and irrational thinking (no Stoic at all), since after all there still some hope.
Do not engage in the premeditatio as a visualization exercise any more. It is clearly not working for you, and indeed it is making things worse. Consider, incidentally, that that’s not how the ancient Stoics did it anyway. They reflected on the possibility of bad events, talked to friends about it, or wrote it down in their diaries. There is no evidence that they engaged in any sort of visualization exercise.
The latter approach is actually an innovation of Cognitive Behavioral Therapy and similar techniques, as explained in Don Robertson’s book, Stoicism and the Art of Happiness. In fact, imagery and “imagery rescripting” are common tools in modern therapy (see here for resources). But they don’t work for everyone, and the one you are attempting is particularly dangerous, since it concerns a very traumatic event, like the death of a loved one.
Instead, I suggest going milder on yourself. Try some journaling, in which you write your thoughts about the situation. Not focusing just on the possible worst outcome, but actually analyzing what is going on, from the medical to the emotional aspects. Update your diary with the pertinent medical information and what it likely means for your mom and you. Write out how you are interacting with her, what thoughts come to mind, what precious memories the situation is triggering. Also, read about other people experiences similar to yours, though try to avoid the “positive at all costs” crowd (see this advice column I wrote about that). Write letters to friends, in the manner in which Seneca wrote to Lucilius, detailing not just your emotional responses, but how the current situation is a test of your Stoic philosophy.
And consider that, as you say, Stoicism is already helping you a lot. You know about how the Stoics thought about death and grief. Just as a partial reminder, here is what Seneca writes to Lucilius, in Letter LXIII, usually entitled “on grief for lost friends,” but which of course applies precisely to your situation as well:
“Let not the eyes be dry when we have lost a friend, nor let them overflow. We may weep, but we must not wail.  … Let us see to it that the recollection of those whom we have lost becomes a pleasant memory to us.  … Fortune has taken away, but Fortune has given. Let us greedily enjoy our friends, because we do not know how long this privilege will be ours. [7-8] … If we have other friends, we surely deserve ill at their hands and think ill of them, if they are of so little account that they fail to console us for the loss of one. If, on the other hand, we have no other friends, we have injured ourselves more than Fortune has injured us; since Fortune has robbed us of one friend, but we have robbed ourselves of every friend whom we have failed to make.  … It was just because I did not do this that I was unprepared when Fortune dealt me the sudden blow. Now is the time for you to reflect, not only that all things are mortal, but also that their mortality is subject to no fixed law. Whatever can happen at any time can happen to-day. ”
What Seneca says about having friends to console us applies also to your family, to whom you should turn for support and consolation, without fear of being a burden. That is, in part, what they are there for.
Moreover, you are focusing on living in the here and now, appreciating every moment you spend with your mom, which is the right thing to do. I have lost my mother recently, to cancer. Several years before I had lost my father, also to cancer. The way I handled the two situations, however, was very different precisely because I had started my Stoic practice shortly before my mother passed away. As you know, regret is not a Stoic value, since the past is outside of our control. But in a sense I do regret not having had the same tools with my father. I did not take his condition seriously enough, not in the sense that I didn’t know, rationally, what was going on, but that I did not internalize the meaning of it. As a result I kept postponing visits to Rome (I was living in Tennessee at the time), and when I did go it was always for very short periods of time. So I was still, somehow, caught unprepared when it finally happened.
With my mother, however, it was a very different story. I was aware, paying attention, focused on what was happening, and doing my best to be near her. Mind you, I did not have a particularly good relationship with either of my parents, so I’m not trying to paint an idyllic picture here. But I am still very glad that Stoicism helped me both to be present (mentally, not just physically) during my mother illness, and to put it in perspective, preparing me to deal with it, as Seneca says, by weeping but not wailing. I sincerely hope it will do the same for you.
P.S.: during a discussion over at the Stoicism Facebook community someone suggested this useful summary of different methods of negative visualization.