Why is writing so hard?

I spend all day writing for the company I work for, but in the evenings, when it's my time to write, I struggle to get anything out.
Stress, tiredness, aching fingers... I've used all of these as excuses as to why my writing has been virtually non-existent of late. The main problem I come up against is ideas. I have hundreds of ideas for stories, loads of characters floating around in my head; some brilliant, some er.. not so brilliant. Often I'll have an idea, think 'Yeah, that's great', start writing a few chapters then... nothing. I lose interest/inspiration and it just fizzles out because althought the idea may be good, I have no idea what story I am trying to tell.

Last year, in my 3rd year at university, I wrote the most amazing story (if I do say so myself!) I was awarded a First for my creative writing dissertation, and I was (and still am) so proud of the story I wrote because I proved to myself that I could make my ideas work. It's now about 10 months later and I've written nothing of any significance since finishing my dissertation.

And I call myself a writer!