Confidence is a feeling that you develop through experience. The more you do something badly, the worse you feel about doing it again, so you burn out, as you have described here.
Say, if you were to try again and things don't go as you want, you'll again go back to not wanting to try again.
To first address "feeling enough", there's no inherent difference between someone who feels that they are enough and someone who feels they aren't. It's more about understanding who you are as a person, seeing your good qualities and your bad qualities. Sure, there may be 5 reasons as to why you aren't a good sub, but there may be 5 others as to why you are a really good sub. To go further, maybe there are people out there who wouldn't really see your flaws as "flaws" and think that those qualities make you more human and endearing!!
Now, if I were in your position, I would talk to someone about my insecurities (delving into my experiences and how they correlate to my current state of mind), and what I can realistically provide as a person in a dynamic rather than what I "should be able to" (aka how much a person who is enough can do).