We all have felt 'not good enough' at least once in our lives, whether it be not good enough for our job, our significant other, our family, our friends, or for any other reason. This is for the people who do not feel good enough right now.
If you do not feel like you are good enough for your job: you got that job for a reason; you are there for a reason. Your boss hired you for the skills and positive aspects that you can bring to the table. You are a hard worker and if you weren't, you would not be working where you do anymore if at all.
If you do not feel good enough for your significant other: they love you. If they did not love you, they would not be with you. If they were not happy and wanted to be with someone else (or just single) they would be gone. But since they are still around, cherish them and the time that you two spend together.
If you do not feel good enough for your family: they love you just the way you are. Besides your soulmate, your family are the only people who would not change a single thing about you. In their eyes, you are the best human being that you can be and they will never give up on you.
If you do not feel good enough for your friends: they love you just as much as your family does. Like your family, they hold your best interests at heart. Your friends love having you around and you can count on them for anything. That's what friends are for.
If you just generally feel that you are not good enough: you are. No matter what your inner demons tell you, you are good enough. You are worth it. You are a beautiful person and you can do anything that you set your mind to.
For anyone feeling not good enough for whatever reason, you are one of a kind and you do have a purpose on this earth.