Trying to believe that I'm good enough

Not sure if this is in the right place. Feel free to move if not.

I need help accepting what I’ve done as good enough. Last weekend I had a bit of a crisis that I’m no good at anything and that the last several years I’ve spent studying and doing sport have all been for nothing. I think that part of the reason for this is that I find it difficult to accept praise, e.g.

- Someone says “Well done” for completing a task at work, but I don’t think I deserve it, because it should have been to a higher standard and I should have got it finished sooner
- If I go for a run and someone is impressed, I downplay it because (in my mind) I should have run faster or further.
- Someone might say I’m doing well after moving away from home and managing on my own, but I don’t think that’s worthy appreciation, as literally millions of people do this and it doesn’t seem like a special achievement.

Confusingly, although I find it difficult to accept praise, I also feel that I *need* validation from other people. I want other people to tell me I’m doing well, but then I don’t believe them when they do. I realised on the weekend that there is actually no one, whose validation would make me happy – not my mother (or other family), not my manager, not my partner (I don’t have one), not my friends or housemates or colleagues or training partners. I also get that it isn’t healthy to rely solely on other people for positive affirmations. Clearly, it’s not about having recognition from other people, but accepting what you, yourself, have done as positive achievements.

My question now is how do I do this? I have spent so long thinking that I should be better and that, while whatever I might have done before is ok, I need to do more to be happy. I’m realising that point will never come with this attitude. I could double my salary, buy a house and run a marathon, and still not be happy, as I feel I should have done more. What steps can I take to feel better about myself?