Hi Helen,
I understand exactly how you feel.
I'm not going to say don't be angry, or anger does no good, or anything anyone who has not felt true anger may advise.
All I would say is - harness it in some way that makes sense to you. Like anything powerful, anger can result in bad things, or good things.
In dementiaworld we often have nothing to blame - the doctors have little clue about the condition and certainly can't arrest it for long, the person with dementia is in their own hell and we can't justifiably blame them, I won't talk about a god as I leave that belief to others, ..... so we can't easily vent our anger, and can so easily have it turn in on us.
Turn the anger into something that enables you to let off steam, to forget for a while.
You say
my 17 year old son was saying goodbye to someone that he didnt feel that he knew
. Oh I know I'm weird, but if nothing else worked [even if everything else worked, maybe], I'd be throwing myself into something that could help him feel he knew her a bit more.
Perhaps your recollections, linked to any pictures you might have, anecdotes, places you may have lived with her, etc. Computers, like anger, can be used for good or bad. If you use your computer to bring your mum's past into focus for your son, I'd say it might be rather positive. Even more so if you were able to do it together.
Shoot me down in flames if I'm way off course and yes - I do know you haven't all the time in the world to do it...
Most importantly of all, please take care.