I have 2 kids.
One was absolutely fine sleeping alone from the off, she was fine in a cot alone, would happily sit watching me do stuff from the floor or a bouncer. She'd try it on a little as a toddler and I'd gently ignore it because she could self-sooth just fine and just fancied stringing bedtime out a bit longer.
The other was a terrible sleeper. Absolutely hated sleeping alone, couldn't self-sooth, nothing I tried worked, hated being put down as a baby, needed way more reassurance and interaction through every stage.
I didn't do anything radically different with either of them, they were just different individuals and have very different personalities.
The bad sleeping one is now 6ft tall with a beard and sleeps just fine on his own.
Your experience with your kids was your experience of your own individual humans, you didn't do it better than everyone else, and they didn't do anything wrong because their children were different.
You don't get to take credit for all the things that worked out well with your kids, just like everything which goes wrong with them isn't all your fault either.
We all do the best job we can with the children we have, it's best to try not to sit in judgement of other people because no-one does everything perfectly.
If you were struggling with something, you'd hope other people gave you helpful advice and support - not say things like - well it's all your fault and it's not fixable now. That's not even true!
OP will muddle along and sort it out best she can just like we all do.