When I feel this way I like to take a mental journey back to when I was first inspired to start painting/drawing/creating -- things like rewatching an old film, or looking through a favourite artist's gallery, rereading a book or listening to music which really inspired you.
Normally I feel like this when I feel like I'm stagnating :( Look at your contemporaries - see what they're making! Or read some tutorials with new techniques!
I think I read somewhere by an artist I watch on the net, that it helps to have more than one painting going on at once - that way you can switch between them when your motivation starts to dry up.
Your piece at the beginning of this post looks really great I thought. It just needs a fresh look at it - I get the feeling maybe you were getting frustrated, and focusing on small parts close up, rather than looking at the picture as a whole and zooming out for a short while.
I hope you rediscover whatever it is you need, and maybe you can share with us whatever you find! Good luck :D