Practice! Like you’ve had to!
You are feeding into their anxiety by never leaving them - it confirms their belief that they have to have you there to be safe.
We need mummy to be safe > so you never leave them > We must have been right, we do need mummy to be safe.
Stop it. They need to see from experience that they can be safe without you, with a different safe adult. The only way to do this is to keep doing it. They need to believe they will be fine without you.
Build up their tolerance. Wean them off of you as their only safe person. Go out for one hour one evening before bedtime. Tell them “mummy’s going out for one hour, daddy’s here whilst I am gone, and you will all be fine”. They will likely be upset etc but it’s just an hour, your DH can cope, and then you’ll get home and say “see, we missed each other but you were all fine, just like I said!”.
Do that a few times, once a week, then start to extend it over a bedtime, “mummy’s off out now, I will be three hours so you will be asleep when I get back but I promise I will come and kiss you in bed when I get in. You’ll be fast asleep! Have fun with daddy like you always do”.
Once you stop reinforcing their anxiety and demonstrate that they are safe without you, and your DH has some practice, life should get better for all of you!