Probably a common trait for artists, having a mess of ideas in your head and finding it difficult to pin one down and give it your best shot. I find the starting out is the hardest part, once I'm engrossed in it then it flows and it's difficult to stop!
One thing I think which helps me is having a dog. Really! I have to take him out every day, no matter what the weather, for about 2 hours. Usually walking on the beach or across the heaths and marshes with no company but my thoughts, I use this time to hone my ideas and visualise what I hope to achieve and how to go about it, coming back with a definite plan and renewed enthusiasm.