You're being far too hard on yourself OP. Children with certain needs make the parenting appear bad at times, but only to those who have no understanding.
My eldest has ADHD and needed a lot of extra effort with parenting when younger. Certain parents I knew made it clear they thought I was useless on occasion. Two dads of my acquaintance would "helpfully" tell my child off for me (child took no notice of course and I was managing it in a way they didn't understand).
I've worked with children all my life, including those with SEN and huge behaviour challenges but nothing was more difficult than dealing with my own child's behaviour while sensing the disapproval of others - though I learnt to live with that / get over it.
My child is now early 20s. Yesterday I bumped into their former childminder who coyly asked me if they were still badly behaved. I just laughed and said they were pretty sensible nowadays - I wasn't upset, I would have forgotten about it if I hadn't read your post.
Tips - make life as easy on yourself as you can. Don't attempt to do what 'everyone else does' if you find it too challenging. I remember reading in a book about ADHD that if you're wondering why everyone else's child behaves well in the supermarket, then it's because most people who have children who don't are NOT taking them there - helped me a lot.
Also routines can be imposed - have your own. With young kids I did. Same event at same time every week for example, or every day. It helps the child to anticipate what will happen next and predictability is comforting.
Do only what does work well. If waiting for your other child, don't expect the twins to sit and wait. Take them for a walk or to the park or whatever.
Remember every stage passes. You won't be dealing with the exact same issues forever. New stages can be difficult, for sure, but nothing lasts forever. It will get easier and none of us have all the answers (though maybe those smug parents who have children who are naturally compliant think they do!)