It's really hard to be happy as a designer. You can never (the double emphasis there is intentional) be satisfied with your design skills. Any contentment you experience with a design you create only lasts 1-3 business days until you see a cooler design and wish you could start all your projects over and make them better.
Everytime I look through my portfolio I say to myself:
"I can't believe at one point I actually thought that was cool. If only I could design that project starting now."
So what impact does this mindset have on a designer's life? Well, first of all, you have to smother yourself in cool things to ignore your constant self-disatisfaction:
Once surrounded in awesomeness, you spoil your temporary self-boost by surfing the net and reliving the shame of your ignominious skill set by viewing the pristine designs of Veerle, Cameron, Mark, and Wolfgang.
So what's a designer to do? Even your next design that's bound to blow the socks off your cat will be out-performed within 1-3 business days. I suggest that the only thing to do is remember that you actually get paid for this. Yes, all this fun, all these toys, all this design lust is profitable. Sweet nectar of life. If I were as cool as the NBA, I'd introduce some really cool slogan right now to express my contentment with this line of work like:
"I love this game!"Instead, I'll just say that design is wicked awesome.

I think you are exaggerating!!! I'm sure you're a good designer!!! You shouldn't forget one thing: when you love and appreciate your works, everybody will do it!!!! As for me, I'm only planning to be a modeller and even now when I can do nothing, I know that my creative ideas are the best!!! Only this way you'll achieve your goal in life!!!! May be you'll here later about me, who knows?! :)
I know exactly what you're saying. I do it on a daily basis.
Overall i'm a confident designer, and i can see myself getting better and more efficient everyday, but its never good enough. its never as cool at that new site i found last night.
My favorite part is working on a design for 4-5 hours straight before leaving the office. Then you come back in the morning and look at it. if you like what you see then you've done well.