You were that perfectionist teacher's pet we all rolled our eyes at in high school, weren't you?

But you have a hard time taking action or finishing things because you're good at everything, but everything you do must be absolutely perfect.

So it appears you understand what the problem is. If you have too many options, its hard to take action because of indecisiveness, and taking action is even harder when you're expecting yourself do perform perfectly. This mindset leads to total paralysis.

My unsolicited advice is to maintain your honesty and integrity, but stop rejecting yourself. If you get a job offer and you feel you're only 20% qualified, express that. Tell the person offering the opportunity that you feel you are qualified to do x, but don't feel knowledgeable about y. But here's the key: if they leave the offer on the table anyway, TAKE THE DAMN JOB. It is your job to be honest, not to disqualify yourself. It is their job to disqualify you. If they still want you for the job, it simply means that they think you will still do well enough, will learn what you need to on the job, and/or they like you as a person, which many employers will tell you is often more important anyway. Also, qualifications are often written by HR personnel that don't know jack shit about the job so be skeptical about them.

Think of it this way. If you're on a date and a guy says that he's looking for a girl that loves riding horses, but you don't and you say so, it doesn't make sense for you to end the relationship if he wants to continue. It just means that he doesn't actually care about that qualification so much because you're worth it anyway. If people think you are valuable stop arguing with them silly girl.