Oh my gosh, I feel the same way sometimes. I started sewing in high school and made quite a bit of my clothes during my early 20's. I gave it all up for cross stitch and needlepoint and am now back to quilting in my late 40's. I look back at the clothes I made and have absolutely no idea how I made them, I'm drawing a blank and am in awe that I was able to do it.
The quilting came slowly to me because I don't like to read instructions very well (heh heh). Breathe, put it aside and start something small, once you finish that you will feel better that you accomplished something and then go back to the big project. I have a closet full of those big projects that frustrated me.