I used to have this problem a lot, and it still bugs me at times. However, I discovered I could get round it by writing a rough text riding the first wave of motivation. It needs to be in proper form, notes won't usually do.

I can then leave it for a while and get back to the text in 2-5 days, regaining about 75% of the original enthusiasm. An additional advantage is that this method of revisiting tends to produce more coherent texts, too.