I would recommend trying to forget completely about describing a characters visual appearance. Instead, try to build characters from the inside out. They need a personal backstory and personality more than your readers need visual hints at what they look like.
Dr. Moon made thick, heavy strides through the snow piling about him. Beyond what his freezing eyelids could perceive, he already knew that the morning blizzard had wrapped a white barrier between his surgery and his home on Frosterly Hill. Perhaps later, he might have to call to announce that he would be snowed in overnight.
“Mercy,” he whispered to himself.
To some it would be an inconvenience. However, to Dr. Moon, being marooned in blizzard with a tin of biscuits for company, would be a blessing. Anything, after all, was more preferable than an evening spent with his insufferable wife, Maud.
“If you went private,” Maud would grumble for the sake of grumbling every winter. “You wouldn’t be expected in under these kind of conditions. I told you. You should -”
But that was the problem. The idea of being trapped with Maud for twenty four hours without distraction was utterly terrifying.
“Fullford, I’ve just remembered.”And: “Darling, I’ve just realized that now might be an excellent time to talk about…”
In an ideal world, Dr. Moon sometimes dreamed of having a shrink ray. With it, he would shrink Maud, mid-mindless banter, to a hundredth of her size. Once small enough, he’d then scoop up his tiny wife in a jam jar. After screwing on the lid, he’d then drive to somewhere quiet and peaceful by the Avon river. There he’d gently tip her out into the long grass.
“Darling, I’m dreadfully sorry,” he would announce, watching Maud climb the longest stems in a bid to reach him. “However this whole marriage business was an awful mistake. I want you to be happy, I really do. But it was either this or some kind of skillfully orchestrated motor accident.”
Maud would love living by the waterside. There would be all of the natural surrounds which she dreamed about when imploring him to buy a Park Home. That said, Dr. Moon would never be able to un-shrink Mrs Moon. Should he, she might find her way to a divorce lawyer.
I write things like this to flesh out characters, even if I decide not eventually include the actual text in a final story. When you do this, you also get to imply certain physical qualities without explicitly listing them.
That’s how every writer feels! If you don’t already, you might want to start making sure to never write in a linear way from beginning to end. Instead, try writing chapters as standalone short stories featuring your main protagonists in new settings and plot archs which don’t even fit into the idea of your overall story. This can help prevent your descriptive powers from going stale.
Also, whenever you get really stuck, kill someone. Nothing refreshes a stale plot like a sudden unexpected murder.