Op you are not a failure. I was you not that many years ago.
I too was had to be there for my mum, emotionally and I wasnt even that old, think I was really young when I understood my role within the family, I was probably around 5.
You are capable of more than you realise. Looking after DC who have challenges in life is so rewarding, and I bet you have learnt millions of things most people would never get, like having to read DC when they are in a meltdown. To navigate the outside world as its seen by your DC, and then over explain behavior most parents frown upon, or turn their nose at, "I would never do that etc comments, I had them and still have them as an adult that is also ADHD.
I bet you wake up tired, worn out, and like you said, generally bored. Because your batteries for self catering are running on zero. And has been for so long you have, like you said, for gotten who you are.
It takes time to get to know you after having been on 100 miles an hour for years, you know the role of being a mum, partner and caterer, but you dont know who you are, and it feels super weird to suddenly go "erm, I need time for me. Like what? But who is going to to this, that and the other?
They need you, and that is that, so you continue on autopilot till your mind says nope. And by the sounds of things that is happening right now.
Its wake up time for you, and you deserve life that is for you. Not the kids, not the partner, but you. And no guilt over that is going to serve you, it will just make you dip into that codependency role and you, one day, will wake up at the age of 60 and think, my kids are grown, and moved out, my hubby has a hobby, and what do I do now?
You dont need permission to have a life outside the family dynamic, its healthy to want something for you, really sodding good, and it takes a whole lot of courage to get on here to voice your needs, so well done. No need to feel guilty for wanting to put yourself first too, and I am sure there are many on MN who will second that. You are entitled to your own needs too. :)