been there many times. a lot of it probably comes from low self-confidence, but another thing is that if you spend long enough looking at something, you will eventually get sick of it. staring at the same picture for four hours straight, getting frustrated by problems you don't have the technical skill to really fix, will inevitably bring about a level of resentment.
but! i can very definitely say that i appreciate your art and the work you put into it. and, at the very least, know that you're able to recognize your own skill level and pinpoint what exactly you need to work on. some people are either ignorant of their shortcomings or just straight up deluded. we're all constantly improving!