I have been and am a good mum to my younger children (diff hubby diff kind of life makes it easier) I was not such a good mum to my older two who as I mentioned earlier are 20 and 21 I have apologised to them but they laugh it off as I tell them I tried the best I could but know I failed them in many ways.
My oldest saw what the marriage had done to me and my ex and it was not nice so understands more however my younger one is now is prison

he has mental health issues, I tried really hard with him but the marriage situation was terrible my ex would not leave no matter how much I begged him, no matter how much the marriage counsellers advised him to go.
He would not leave his house. Things that made life for him nice.
So they the children saw and heard lots of arguments (I am no pushover and gave as much as he did which looking back I was wrong) it affected them badly my eldest adores me but is constantly worried that I am depressed no matter how much I tell him I am fine he worries constantly not a good thing for a young man to do about his mother.
My younger one resents me for leaving the home 10 years ago and for leaving the area four years ago (even though he was at that time in prison) he resents my new hubby again I dont know why as he has been a good step father to him for the past nine years.
He hates his real dad but loves him too so is very confused and I am having to constantly explain how sorry I am for failing him and he accepts it but in a month or two brings it all up again and I am yet again saying sorry. (I will do this for ever if I have to) In the end I love all my children the same but feel guilty about my lack of parenting skills to my older two.
I have to take on my current hubby their step dad who for whatever reason feels I have said sorry enough and gets frustrated with me I dont care he will have to get over it...
I have rambled on here to you but I guess I am trying to get over to you never give up on him or your relationship with him it WILL get better, swallow any pride and let him know you love him and that yes you failed but no matter what you still love him and he will always be your child the child you adore.
Write him a letter if you want to or tell him face to face when the moment arises. Just dont leave it too long as you are both missing out on what you could have !
Hugs to you and I wish you well.