Green Finger issues

I love gardening and I adore plants. In fact I would live in a forest if I could.
But there's one thing holding me back from being green fingered and that is my inability to be green fingered. I only need look at a plant and its roots shrivel and before I know it my plant has died.

I either end up overwatering it or It dies of dehydration. I just can't seem to get the balance right whatever I try.

Does anyone else have this issue?

Also I am interested in a plant that is low maintenance, quick spreading, flower based, loves rock, chalk soil, semi-shade areas and is cheap and available.

I have a rockery which I built in the garden area and I need something pretty to spread over it and fast, to give a pretty look.

Anyone know what I could put there, which will last most of the year round and will come up year after year?

I'd love to know, thanks in advance.