I was born with giant omphalocele, I'm 21 years old, I feel bad ...

All these feelings are perfectly acceptable in front of the physical defect you have, the most important and serious is that you are not managing to deal with this situation, hiding with a top or sweater does not let others see, but you see (and feel), you probably have more self-repudiation than other people have repudiated you. I'll give you an example of the situation I want you to understand: has any boy ever stopped fucking you about it? I doubt it. You need to start accepting your problem as a blessing, if it were not a scar you might not even be alive. Physical appearance is not the main thing in a relationship, if it were there would not be beautiful people single, alone, separated and unhappy. Look for a psychologist and start doing therapy, you have to "work" it.