Hi Mark
I've just back-tracked over your previous postings to get more of an idea of your family situation, and I certainly don't think you're going mad!
I think pretty well all of us feel knocked off-balance with being presented with this awful disease, and the pain of knowing that there is no cure, just medications which will (we hope) slow down the effects of it. Whether we live with the 'patient', or we live at a distance and someone else does the immediate 'caring', we feel we should be able to do something more about it. It's natural, because we love them. You've probably seen references here to "the guilt monster" who haunts us, however well Mum (or whoever) is cared for. You always feel you could or should be able to make it better, however illogical that feeling is.
We all react differently to the stress. Some operate the stiff-upper-lip mode, some are in denial that anything's wrong, some cope brilliantly all the time (except when they nip out to the garage, shut the door & have a good howl). The rest of us get things right some of the time, mess up at other times, and generally feel pretty miserable a lot of the time. If you have a pre-disposition to depression (as I have, & probably several other contributors here) having Alzheimer's in the family isn't going to improve it for you. If one was not clinically depressed before this situation arose, now is probably the time when you would be likely to start learning how it feels! This is something you can get control of, by going to your doctor & explaining the situation and how it is affecting you, and asking for help with it. It's related to the situation with your Mum, but it's your medical problem and you have to deal with it if you hope to be able to play a part in your Mum's care, whether that's to be at home or in a Nursing Home of some sort.
Relative to the Birthday thing, family occasions are painful. Soon it will be Christmas, which is my personal idea of hell anyway. Everything's supposed to be so perfect and full of goodwill etc., and it NEVER measures up to the ideal, even if there is no complication of illness to contend with. Anniversaries, holidays - we have to get used to the idea that they may never be the same again, and try to support other family members who will be feeling the same.
I'm sorry Mark, this is really tough on you, and on your step-Dad & sister. If you are only 28, I would guess that your Mum is quite young (i.e. under 65) to be suffering from Alzheimer's Disease.
Best wishes, keep posting if it helps to get some of the distress out.