I'm very pleased that your ds has recovered from his recent op. Is he back home now?
It's my understanding that during term time your ds is collected and returned to your home daily, with the exception of the one night a week he stays over at his school. It is unclear whether you receive any other assistance in caring for your ds during the week or at weekends, but presumably you have access to additional help during school vacations times as both you and your dh are in full-time employment.
On one of your previous threads you mentioned that your ds goes on a residential holiday for one week a year, albeit that funding for this year may not be available.
From what you have said, your ds's physical care needs are shared between yourself and your dh and, to that extent, they are not as onerous a burden as they would be if, for example, you were living in a country where little provision is made for children with disabilities.
However, with regard to your mil, it's clear that she is suffering from either senile dementia or Alzheimer's and it's, therefore, not within her power to halt the progressive loss of her cognitive thinking, or control her resultant behaviour any more than your ds can stop his disabilities retarding his development or control his double incontinence.
By virtue of the fact that his siblings do not live within easy travelling distance of their dm, the burden of care for your mil has fallen to your dh and it is likely that the current status quo will remain for the foreseeable future.
I am glad to see that SS are now on the case, but you should be aware that any care plan that is put into place while your mil remains in her own home is likely to consist of morning and evening visits from a home care worker. As such, this will not prevent her leaving her flat whenever she wishes and approaching neighbours/strangers, or arriving at your home unannounced at inconvenient times.
All things must pass but, until they do, effectively it comes down to the serenity prayer which is not an appeal for anything other than the serenity to accept that which can't be changed, the courage to change what can be changed, and the wisdom to know the difference.
The needs of your ds and your mil cannot be changed, but you can change your perception of what you currently see as unjust and intolerable burdens by recognising that the anger and resentment you have been holding onto for a very long time is causing you as much, if not more, stress than the challenge of meeting their needs.
It may be unfair that you have to put your plans for further education on hold, it may be unfair that you are working in a job that you feel is beneath you, it may be unfair that the bright future you hoped for has to be put on ice until such time as you can pursue it, and it may be unfair that others appear to be gliding effortlessly through their lives.
Be that as it may, if you can find the grace to accept that you cannot have it all at the present time it will be possible for you to reach an accomodation with yourself whereby you look to gain personal satisfaction from concentrating on what needs to be done to meet the varying demands on your time and energy, and on those of your dh, rather than raging at the unfairness of fate.
I haven't written these words lightly as, having kept an aged relative who suffered from senile dementia in her own home for some 10 years while being a single parent to my dc and furthering my demanding career, I know full well that being a 'carer' is a thankless task that can take one to the very brink of the abyss - and to the bottom of it.
Although I was fortunate in that I was able to employ a small army to keep my own domestic show and that of my relative's on the road, neverthless, the challenge of keeping the plates spinning while juggling with seemingly never-ending tasks fell solely on my shoulders and many's the night I sobbed myself to sleep from sheer exhaustion and from what seemed to me to be the utter hopelessess and futility of it all.
If your mil has anything in common with my aged relative, I'm sure she didn't set out to be another yoke around your neck and it's likely that the poor lady believes there's nothing wrong with her and that she's coping admirably, while her nearest and dearest are on their knees with worry about her welfare.
Try to stop giving yourself such a hard time, honey. Find the flow and go with it - and please keep taking your medication because your dh doesn't deserve a repeat of what happened when you decided to play russian roulette with your epilepsy drugs.