Adults are often way too hard on themselves in their creative endeavors. I saw this in my Wilton cake decorating class, and I deal with this myself.
One significant factor is that, in our normal lives, we don't have much experience being bad at stuff.
I grew up a perfectionist and school was very easy for me. Because of this, if I didn't think I would do something well, I didn't do it at all. This is very limiting, and can make you boring.
It was as an adult that I developed a self-image that wasn't dependent on being good at stuff. Consequently, I am free to do things I'm terrible at. I may find a new activity I enjoy, but even if I don't, I've stretched myself and I may have even had fun.
The other main factor is selection bias. On Pinterest or on television (Ace of Cakes, Cake Boss), we are seeing amazing work done by professionals who have devoted a lot of time, effort, and resources to their work. These cakes are on television or shared online because they are extraordinary. The same is true for art and writing.
I am a regular person. I don't have a lot of resources or years of my life I can devote to baking and decorating. My last art instruction was in middle school (age 14, for those not in the US). I quite clearly see the flaws in the cakes I make. It would be very unfair to compare myself to an experienced professional, but that is what I am tempted to do.
People, especially kids, are excited to have a a homemade cake. I have had no one, ever, complain about an ugly cake, and I have had some disasters. The time and attention you put into it are often enough; attractiveness or beauty are an unexpected bonus.
If you are feeling down, go to Cake Wreaks. These "wreakerators" are professionals, and you are definitely better than they are.
This isn't a job or a competition. But, even if it is, that doesn't mean you have to be miserable.