Never feeling like you're good enough?

I have a constant, persistent, nagging feeling that I am just not good at anything. People will compliment my art or personality, and I feel obligated to say "thank you" even though I have the knee-jerk reaction to deny it. I feel like my art --despite logically knowing I'm good for my age-- is just horrible, my personality sucks (because I know all my flaws), my looks are... well... not pleasant to say the least, and really that I'm just a f-up and nothing will ever change that. My writing even sucks.

The worst one is more to do with other people, though. If I'm around someone who's in a bad mood, or depressed, or sad, I'll try so hard to make them feel better without imposing or pushing or being creepy about it, even though I know from personal experience that trying to make people feel better normally doesn't work. So when it doesn't, I dive into this pit of despair and just go into a cycle of "Why can't I help?" "Am I not good enough to be a friend?" "I hate seeing you so sad" "Why isn't there anything I can do?" "WHAT IS WRONG WITH ME?"

So it goes from feeling horrible for the person, to despairing for myself. Sometimes if it's severe enough, I'll get into the how-dare-you-pity-yourself's but that's another story.

It probably doesn't help that I'm reminded of every single flaw I have when I'm at home.

This whole thing actually stemmed from a post from one of my friends on Facebook -- basically my best friend who actually knows the extent of my esteem issues, depression, self harm etc and is the only one actively helping me fix myself. She moved here at the beginning of the school year and I know she misses her old town, and she posted a status about having mixed feelings toward moving and that she missed her other friends even though she met some awesome people along the way, and how she's stuck in a rut and stuff over it. My first thought on this was something along the lines of, "What, I'm not good enough? Isn't it my job as a friend to make you feel better? What's wrong with my friendship?" even though logically... that's not the right response. It makes me feel selfish and egocentric that my first response is about how I feel instead of how SHE feels and I hate it.

I'm sure some part of it stems from my needy/clingy/jealous behavior. I get very jealous when friends fail to acknowledge me for any significant period of time, or don't spend as much time with me, and I truly crave attention from my friends, which tends to make me very, very bitchy when they're not around and then blow up on them later... not fair to them at all, but it's also not fair that I have to be practically glued to my friends' hips all the time.

Is there a good way to deal with this kind of inadequate feeling? Like I'm just not a good enough friend? Because I'm all out of ideas. :\