What did I do wrong?

[Ger-annie-um]

Yeah, that's definitely spider mite damage. Mine was exactly like that.
You'll want to check if the mites are still there (they may already be gone). Check the undersides of the affected leaves. If you see very fine webbing then there are still mites there. If you don't see any webbing, look for tiny brown specks on the undersides of the leaves, and if you rub the leaves they should leave small brown smears on your fingers. Those brown specks are the spider mites.

Assuming your plant is still still infested, ideally you should spray them with a pesticide that will kill spider mites (most do, but not all; check the bottle before buying it).
If you don't want to use chemicals, move it outside now and leave them there unless any nights under 4c are forecast, as the mites don't like outdoor conditions as much. Also try spraying the leaves a couple times a day with water, as the mites don't like high humidity.

And whether you still have the mites or not, and however you choose to tackle them, give the plant a good feed with a general purpose feed (like growmore, poultry manure, or blood, fish & bone), and also give it a liquid feed with a high nitrogen feed every couple weeks. This will encourage the plant to grow plenty of new leaves.
The mottling on the old leaves is permanent and will never recover, but they still function mostly as they should, so don't remove them. The new leaves should not be mottled as long as you get rid of the spider mites (if the new leaves turn mottled as well that means you still have the mites).

Ameno
thankyou so much for your advice.
I'm a complete novice when it comes to citrus trees! I've sprayed the tree with an 'ultimate bug clear' which I hope will help, I've also put some growmore on. I have a vitax summer citrus feed to use (weekly?) Hopefully I will soon start to see signs of life. Oddly the lemon I'm trying to grow from a pip given to me by a lovely man in Pathos is doing pretty well, although it did lose a lot of leaves it has new glossy ones now and is growing well ( probably won't see lemons for a good few years though)