Hi and welcome.
you never have to apologise for rambling here.
You have given me food for thought - its not a situation i have been in - although i never knew my dad until i was an adult he was alive but i didnt always know whether he was or not as i also had to deal with tight lipped and negative reactions when i asked questions.
I only met him twice before he died so i also understand there will be things about him i will never know and i can empathise with feeling the odd one out at school because back then a single parent was rare - i was the only one in junior school who didnt have a mum and a dad.
I made things up - because there were those times when you were suposed to write or tell the class about what your dad does - and i didnt know - he was never a prince or anything ridiculous in my stories - there was always an ordinary job but one which meant he had to be away all the time.
Others, adults, did know things about him and only looking back now, knowing what i later found out, can i have any understanding of why i was bullied over it - he was gay,being gay wasnt ok and the "sins of the father are visited on the children" -so even though the bullys themselves probably had no idea what gay was their parents didnt discourage it, i was some-one to be looked down on. Of course he wasnt just a gay man and he wasnt just a father who was pretty much forced to abandon his child he was much more than that - but i dont know in what ways, i know he loved dogs because he had one - i know he was a lorry driver (maybe i wasnt do far off in my long distance job stories) - but thats it -thats all i will ever know.
I,m not trying to turn this into being about me, but just to explain that even in this different situation i do get what you are saying about that side of things - in your case your mother the angel, in mine, my father the sinner.
When he died i felt cheated - because although people seemed to think losing a dad who they had known all their lives was much worse than losing one you didnt know at all, and so i shouldnt really grieve for a stanger - it was the missed opportunity no happy memories and no chance to make new ones - and in a sense i think that is the same for you - because you have that empty void that other people cannot fill - all our memorys are personal and will never be exactly the same as those of some-one seeing the same thing at the same time and place so they can never be real they are always some-one elses story.
I think one course you could take would be to look for counselling you are grieving and it is so very long term because you are grieving not for who you lost in the sense that we grieve for those we have known, but because you can never truly know who it is you lost - so for us we can replace some of the loss with the memorys of a lifetime or much less time, but all you have known has been the loss.
Another could be to go back to your dad and get him to tell the stories again because as an adult you may be able to read more into them than you could when you were younger -they still wont be your stories, but maybe you would be able to know more about her as the real person she was and not just the angel she became - perhaps start with a trait you have, that isnt angelic and ask him if you got it from her. take a circumstance and how you reacted and ask how she would have reacted and thats an opener you could use.
for example my grandson was trying to fix his bike and something slipped and cut his finger so he ended up using an unaporoved word, throwing the tools down and stomping off -later he asked me what grandad would have done -(he was a mechanic) and i said exactly the same as you - and like you he would have come back and tried again until he fixed it - because i want my husband to be a real living person to them, not a patient saint who wouldnt have thrown down his tools or sworn.
Last thing ( see told you rambling isnt something to apologise for) - i would like to think that now, teachers would react differently and encourage you to make mothers day cards - if she has a grave you could have taken them there, or kept them in a special box - or taken them some-where special too her - but it isnt too late to do that, you could, next mothers day, make that card - make it part of a ritual - maybe plant a tree in her memory or follow her football team anything you can think of so that it becomes your own memorial too her - because even though you dont know her, you still grieve for her because you didnt know her you have probably never given yourself permission to grieve for her - and these are the things people who are grieving do in order to help themselves to learn to live with it.