You're most likely to think your writing sucks right after you have a breakthrough that helps you understand your craft better.
In other words, that miserable "I suck" feeling is a sign of progress--it's the normal accompaniment to becoming a better writer than you ever have been.
While I'm sure that is the case in some instances, that doesn't really happen with me. Sure I look at early work and am grateful for how far I have come, but I don't think it sucks, I just think it lacks progress. I suppose my changes in style have come so gradually, that I don't have a lot of "a-ha!" moments. What I do have, is a 47 year old body that is going through menopause. I have days when everything seems to suck, and sometimes my writing takes the brunt of it. In my case, at least, it really seems more of a mood swing than something born of a learning experience.