@beepbeep
thanks so much. I've had ALOT of counselling, some of which has helped, some made the relationship with my mother worse (we wrote a letter about how she made me feel, but obviously it was thrown back in my face as it "upset her so much" - no taking on board any of my feelings!) and some that made me realise how toxic she is!!
I live a long way away from my family now and have a great DH & great kids, but often feel that a huge part of my life is missing and spend time wondering what if...
OK, I'm back...... This is a subject dear to my heart, and I sympathisr so much with your situation.
Again, I do not want to elaborate too much about my life, but there ARE such similiarities that is may be unavoidable. One thing I would say is that, however much people may advise you to "stop [or even start] trying", that pull - and, as you say, feeling of "what if" will always be there, an awful nagging sensation.... BUT (massive But), nothing you do can change the fundamental selfishness and apparent disinterest of these people - and no doubt, they will say, if have not already, that they are "too old for all this" etc etc blah flipping blah
It IS so painful - and this is from someone who is such a masochist that I have spent a bloody lifetime (almost) trying, being forced to tap on the glass like a vampire asking to be let in. I have also been replaced by various children many times over (including a stepchild for my mother) and my daughter by other preferable grandchildren (she was literally taken by me to the other side of the world [NZ] to meet her grandfather (my father) who has never seen her since. She was 33 last month, so the maths is there to see.....
) My heart breaks for her - but she gave up on him a long time ago - and she did not have the loving and secure environment that it sounds as though your DCs do..... That said, she now has two degrees and is a Palliative Care Nurse, so it has not held her back.
Your children can be one step removed from the "rejection" that you feel - and may possibly continue to feel, as you cannot predict what these people will do if you make any or all (and it would be all on your part) efforts. They may well be "too old to change" - but they always were, no matter their actual age.
I may be wrong - as every person and "case" is different, but I don't want you to put yourself in a weakened position or state of mind... But I have had a father like yours - and a mother too, it seems - and I have suffered and been made to suffer - and if I could have my life over, I would have still tried - but I would have stopped trying sooner......
Again, this is my experience and I have not turned out so well as you - in part, by devoting - WASTING - thought, energy and consideration to two pretty unpleasant people. So, I urge you to be careful and give a lot of (but not too much!) thought to how much of yourself you want or are able to give to this - and to them..... They will likely not even appreciate it, I am sorry to say... or understand at ALL how much it means to you or how much of yourself you are offering to them in order to have something you SHOULD have had without having to try 
Regarding letters - I have done this too - and also been on the receiving end of some stinkers (and as emails, which my father preferes, and woe betide ignoring or not seeing one from him). I do actually think they can be dangerous, so would be wary. Someone once said, "A letter is like a hand grenade thrown into a room, and the door closed". And I do believe that, so any of my recent letters (fewer nowadays) have been an exercise in clearing my mind, but have not been sent. It can be as bad to actually send them as to receive them, I have found....usually no good comes of it, or at "best", it was a futile exercise, or the respone - as with your mother - does more harm than good. There are better/other ways to use - and PROTECT - your mind.
x