Everyone does it. Be it about important things like not getting a promotion because you could have worked harder on your last assignment, or should have studied that extra hour for your exam, to something as simple as you could have put more effort into your last post, it happens. Those self doubts translate into most people making cuts at themselves, respectively for laziness, being easily distracted, and underachieving their potential.
Most everyone has insulted themselves at one time or another to a friend simply because they feel bad about their performance or skills. And really, what is the harm, right? You aren't spreading any sort of insults about anyone else. You aren't making comments that makes another person feel bad, only yourself- and really it is not so much cutting you down as an outlet to relieve stress. Right?
What most people seem to forget is that the world is not at all a fair place on any level. Sure, you missed out on your promotion, but there is the person below you waiting to fill your spot when you do get that position. There is the man in your class who is just not as smart as you, even with that extra hour of study. Or the woman who thinks you are far better at writing than she could hope to be.
The world follows a hierarchy in every single aspect. You are behind people and, as little as you may want to admit it when being self depreciating, there is someone behind you. Somewhere, there is someone you look up to, and someone who you may not even know more than the face of, who looks up to you.
And how do you think that person, who thinks you are a hundred times more skilled than them, feels when you say your own work is crap. Your work is crap, and by their basis for comparison, their work is made to be 100 times below crap.
There is no harm in saying you should try harder next time- in all honesty, the person looking up to you is likely thinking the exact same about themselves. It becomes a problem once you say you can not do something. You are cutting off your own ability to do it by a self-fulfilling prophecy, one that gets passed down and affects every person who thinks you are even a little better than them. That is a chain affect. Someone is below that person, and someone below them. You've successfully smashed the moral of some people who you may not even know exist.
So before you start to insult your latest post in the Shoutbox, or your artwork in the Art forum, take a glance at the number of people who use E. If you think it is physically impossible for even one of those people to think you have some ability or trait they wish they had- then by all means, utterly devastate your own work. But if there is even the faintest doubt? Odds are that doubt is justified.