Sounds like you have clay soil, your description of it is very familiar - needs a pickaxe when its dry and sticks to your fork or spade when its wet.

Organic materials, that's the answer, and lots of them - you need to add as much garden compost, soil conditioning compost, small grade horticultural grit, composted animal manures, spent mushroom compost, anything you can get hold of to change the structure of the soil to something more workable. It's not the lack of sunlight that's the problem, its the solid clay soil, and the only answer to that is to add organic humus rich materials year on year. Best dug in, particularly the grit, but all the other stuff can just be laid on top like a mulch so it works its own way in over time.

You can even use grass cuttings as a mulch if you're desperate - not pretty but it worked for me. (Might need a bit of fertiliser to contract any loss of nitrogen but I never noticed the difference) Chopped autumn leaves work as well if you are in a hurry to get started while waiting for your compost to rot! (What an attractive border you have)

I have exactly the same problem so I do sympathise, but if you do as Bamboo suggests you will eventually get a more workable soil. I've been chucking all sorts at it for years and now have a bed that doesn't get rock hard and crack, and is much easier to work.

Great answers, and I find that the grit is just as helpful as the organic matter. There aren't many plants that will survive full shade AND cold wet winter soil.you can get away with a lot if your soil drains well, and clay is generally very rich in nutrients, which is a great bonus.