If you are counting your success by how much less than 100% of what someone else could do in their prime moment you can do, you are sabotaging yourself. First of all, it's not useful to decide your value by comparing your cooking to one of the top 10 great chefs, AND your driving to one of the top 10 great race car drivers, AND your income to one of the richest people in the world, etc. None of those people meet your standard for yourself of being on top of everything--each one is only great at one thing (or a rare few at more than one, but everybody is bad at and fails at lots of things--even the thing they are great at, at times).
Even with ordinary people and abilities, you have to compare whole people to you, not their best quality to your same quality. Mary Jane may be able to make her husband shiver in bed, but can't spell to save her life. Ted may be able to research anything in record time, but he can't walk a mile without getting winded. Whole people are always flawed, and messy in their own individual ways--that is what it is to be human. To have these expectations of yourself is realistic.
Secondly, by this scale, if you did 90% of your goal, it would count in your head as -10%. This means that unless you do something perfectly--100%, you are failing by some percentage. If you, instead, count up from 0% of what your goal is-- a goal that is realistic for you, not someone else--then 90% is 90%--almost all the way to your goal. If you also give yourself lots of encouragement along the way, you will end up doing a better job with much more sanity than you ever could with the -10% system.
Now think about what you would say to your beloved 10 yr old neice, if you were teaching her to do something you know how to do well. Assume you would teach her in your most grown-up, most loving/caring and protective mode. Specifically (think about this as realistically as you can--change the age if you like, and then write down your answers):
What would you say when she tried to do what you said?
What would you say when she made a mistake?
What would you say if she didn't practice one time she could have?
What would you say when she did it just the way you taught her?
What would you say if she were trying, but it was very hard for her, and she felt discouraged?
What would you hope you'd say when she or you felt frustrated with her progress?
Do you have all that answered and written down? Ok, now if they are kind and compassionate and encouraging answers, practice them as often as you can on yourself--start with using them with each step you take toward your goal. These words should feel good--challenging but inspiring, energizing. They should lead you to believe that you can do this, and if you make a mistake or fail, you are just as lovable as if you don't.
You must practice this. This way of thinking is like a new language. You have practiced your way all day every day for most of your life. To change it, you have to practice the new way. For some people, it's helpful to think of parenting their young selves, and they may even use a picture of themselves as a child--one that evokes compassion--to practice compassionate, non-shaming parenting. For others, it's helpful to think of someone in their life, or a character in a movie or book, or someone in public life who seems able to parent in a nurturing, protective, wise way, and not with shaming or disrespect or unrealistic expectations, or even neglect.
Carrying this person in our heads, and consciously relating to the person can help us develop the good parent inside we all need to be healthy, happy and whole. This is what happens naturally, for people who get good enough parenting when they are children--they imitate good parenting until they've practiced it enough to do it for themselves. If you have unrealistic standards for yourself, it probably came from imitating parents who had unrealistic standards for themselves, or for you, or for each other. If you practice being the parent you wish you'd had, you will develop those skills, and finally have that parent at your disposal at all times.