Could you step on sideways somehow, so that you're not looking at the stairs falling away when you're going down, but facing the wall or the other escalator? Or close your eyes once you're on (with someone else with you to say when you're getting near the end) so that you get used to the sensation of moving but without the disconcerting visuals.
You might be able to find a nearly empty shopping centre now to practice on, or a large Boots or something, that is allowed to open. Or perhaps a first step, if the steps on the escalator look overwhelming, would be one of the conveyor type escalators that they have at some places like Ikea or big supermarkets, where you are going up but more gently, and you can hold on to the cart. Or even just the totally flat conveyors at airports.
Then gradually you could get used to little escalators. I know they'll be hard to find right now, but some places I've been have really short escalators - they're more for make sure places are accessible, so newly built buildings where there's a short flight of stairs, but they've put in an escalator as well for people who can't use stairs. That might be just long enough to get used to it.
Or somewhere like Debenhams on Oxford St, if you're anywhere near London, where the escalator is flat for the first 6 feet or so, and only then starts going up. It feels less weird to get on.
Obviously most of these are hard to do in lockdown, but as things gradually get back to normal, you might be able to find more of them, and practice at quiet times, over and over, til you get calmer. And you don't have to like them, ever, but just to be able to do them
I remember being a bit scared of them as a child, not so much as a phobia though, but didn't like that first step of getting on, for long enough to be old enough that remember it. I am fine now.