Practice more Compassion/Lovingkindness. Learn how to take joy with what you've already done, rather than what you think you have yet to do.
It comes back to the practice of the Brahma Viharas. The development of unselfish emotions helps us to let go of idealism and to understand that only the Buddha is perfect. According to the scriptures, even an Arahant, a person fully enlightened, is not perfect in the Paramis. So it's a pretty high ideal to expect that level of perfection of ourselves at this point of time. Going down the levels of Enlightenment, only an Arahant is free from all the hindrances. A Non-Returner still has a level of worry and restlessness, imagine that. A person who is said to have purified the mind from all desire and aversion and who no longer comes back to the world, still has some level of restlessness and worry. Much more subtle than ours of course. A Once-Returner, has Subtle levels of desire and aversion. A Stream Enterer, the lowest level, if you could call that a "low" level, and is free from doubt.
So until we reach the first level of Enlightenment, we need to understand that these hindrances will arise. If we expect them not to arise, we are not understanding that we need to climb a ladder, starting from the first step before you can get to the top step. It's very difficult unless you have a trampoline, and still if you try to bounce to the top step the ladder might fall down. It's good to start at the bottom step and then go step by step. Imagine you are climbing a hill, you try to be in the present moment with each step, try to let go of the wish to achieve reaching the summit.
Sometimes perfectionism comes from self-aversion and lack of confidence in oneself, not having the capacity to forgive ourselves. But if we don't learn how to forgive ourselves it is very difficult to really forgive others. Then if we put our ideals and perfectionism onto others not only do we have to be perfect, but everybody else has to be, too. That includes their speech and actions, as well.
So how can we lessen the tendency to be a perfectionist? Just try to do the best we can, but understand we're all human beings and perfection doesn't usually apply to human beings, except if you are a Buddha. Even then, if you want to be a Buddha, you're going to have to perfect the Paramis over many lifetimes.
So by learning how to forgive ourselves, by having Compassion and Lovingkindness for ourselves will help to balance the tendency of thinking that we always need to be perfect. Also, it is important to learn how to have Sympathetic Joy with ourselves, recognizing our good qualities and the things that we did do well, so that we can see both sides of ourselves, not just the things that we feel we didn't do good enough.
Otherwise, this is when we start judging ourselves for not being perfect. But if we can learn how to open to the things that we did that were good, reflecting on this good Kamma, doing Sympathetic Joy type meditations, learning how to take joy with our good qualities that we already have, and the good qualities of others, we may start to see a letting go. Using Sympathetic Joy and confidence inspiring objects like this are very helpful when learning how to let go of perfectionism, because often it is the times that we are running away from who we think we are, we haven't actually accepted who we are, and we are trying to get to the person we want to be so we can feel okay about ourselves. But if we learn to have Compassion for ourselves and the imperfections that we have and learn to have Sympathetic Joy with our existing good qualities that we already have, then it helps us to be more realistic with ourselves and others, which in turn allows more Compassion to arise for ourselves and others.