What idiom can I use to describe a situation where I didn't do something I should have done at a certain point of time but did it eventually ?
You could use “Better late than never”.
said when you think that it is better for someone or something to be late than never to arrive or to happen.
If the person feels guilty that things didn't go as planned, @Scott is right with 'better late than never'.
On the other hand, if the person felt the schedule was the problem in the first place, there's
as slowly as one wants even though others want one to act more quickly
"You took your own sweet time turning this in."
"Well, [wildly implausible medical/familial/national-security excuse]."
"Yeah, well, you're still losing 20 points."
I must say that the expression at long last (or simply at last) fits your description fairly well too, but whether it is the perfectly fit for your particular situation really depends on the context of the situation which you have not provided:
finally, after difficulty, delay, or irritation
At long last, I managed to finally get it done! Phew... What a relief!