What if I told you that I told my coworker I'll finish the project by next Monday?
Why did you tell him you'll finish the project by next Monday?
I want him to think that I'm very competent at my job.
It's good to be confident in your abilities and to complete your work when you say you will. People depend on you and it's important to complete projects when you say you will. Being undependable will make your coworkers unhappy with you.
He can go fuck himself. I'll never do it on time.
Well I don't think you need to say that towards a coworker. I'm just saying that it's good to get your work done in a timely fashion.