The gardening is an excellent idea even the seriously cack handed (me).
When I was in a similar situation to you about 15 years ago now and split from dh everything was grey. Looking back don't think I was depressed I was in shock.
Anyway, when I got to the stage of trying to pick myself up I thought of cheering garden up. Pots with 'cute' and cheerful flowers did wonders. Normally can't say I'd have taken any notice of flowers at all. But I remember back to that time and going home to check on my little fifi flowertots in the garden did give me a focus.
I think when everything has lost its shine is when really cheerful cutsie kind of flowers are the only things bright enough to notice. Think it takes us back to our childhood when we were smaller, down near the flowers height and seeing everything for the first time. There's something therapeutic about it.
If you're not thinking I'm nuts yet ! Think back to some really cheery flowers you remember from playing ouyltside on a sunny day when you were a child. Or of course, if you're not mad like me then there's plenty of proper grown up flowers too!
When I said thinking back I wasn't depressed, I was in shock is something wanted you to think about. The depression that I've taken medication for is , I think, actually a culmination of different knocks over my adult life that just wore me down.
I think you may find similar with you. Things like the massive shock of you being there trying to save your step father etc. the worrying about your mum etc.
It may be lots of little bumps along the way have led you to feel how you do today. This is why you need to be kind to yourself and not think - oh other people just get on with it. Everyone's back history comes crashing through their protection barrier when another blow comes their way.