I am so sorry, I know exactly how you feel. I wish I could say something helpful, but I don't think I can.
What you are describing is something that I live with every day and I would imagine so does everyone else with a symptomatic cavalier, however well the medication seems to be working.
Owning a dog with SM does spoil the uncomplicated joy that you feel in your pet ( although in other ways it makes them a lot more precious to you ) and I really don't know how you can ever lose that worry.
For me it is the not knowing how much pain they may be experiencing.
I think you do become slightly paranoid, you do spend time wondering if you are imagining symptoms or, on the other hand, whether you are being cruel because you are not acting on your vague worries.
I cannot say that I feel my anxiety about my three affects their behaviour, although their behaviour ( when I wonder if they are having a 'bad' day ) can cause me to feel even more anxious about them and whether I should be looking again at their medication.
I have lived with the situation long enough to appreciate every day I have with them, and to enjoy them when they are obviously having fun.
Scaring pet owners is one of the accusations directed at the Pedigree Dogs Exposed film, and at Carol Fowler and myself when we appeared in the film in an attempt to bring the problem of SM to the cavalier owning public.
I know that has happened, & I am sorry for those that have been scared for no reason, but better a concerned and well informed owner checking for something that may not be present, than a dog in pain and the owner oblivious to the signs of extreme discomfort.
Ruby has an owner who cares, an owner who knows her problems and will not let her be in pain if she can help it.
You are a great Mum to her & she is so lucky to be with you.