Sometimes distress has to be buried deep in one's own heart as there is no other feasible place for it
IMO you're right in thinking that if we are stoical (otherwise known as uncomplaining or heroic) by nature, or because circumstance gives us little alternative, we have no choice but to bury our distress somewhere deep while we get on with what we have to do.
It can also be expedient for us to put on a brave face and spare others from the fine detail of what we are truly suffering/undergoing because it can be so time-consuming to tell them - and each time we tell others of the true reality of the situation, we have to re-live what we've been through, confront our fears all over again and, in some cases, open the floodgates to new ones.
Of course, the more we spare others, the more we take on ourselves until it can seem as if we're looking out for everyone but no-one is looking out for us - but that's an additional price we pay for holding, and keeping, ourselves together in face of adversity.
In your situation, you and dh need to grab every spare minute for yourselves; to recharge your batteries, to take time out, to be able to turn off and pretend/fantasise that your lives are 'normal', or do anything/everything you can to maintain your equilibrium so that your strength can be renewed ready to face the next challenge - which, in some cases, can simply be about getting through the next hour, day, or night.
It's unfortunate that your need and the opportunity to have some brief 'me' time has coincided with your dm's visit to db. When you see dm on Sunday, let her know how much you appreciated her offer to visit and how you would have loved to have had her over, but that this stage of ds's recovery you were concerned that you would not be able to have had the quality time with her that you wanted, and that you would have felt upset at being short-changed. Give her a big hug, tell her you love her, and that you know she'll understand - because she will.
Please be aware that at some point you are going to have to go to that deep place, dig up the distress, and let it go to a place where it can do no harm. In the meantime I wish your ds a speedy recovery, and I wish you and your dh well.