Dealing with estimates as a junior programmer

Question

I have been working for a couple of months now in a company that estimates (for the general population, not juniors specifically) tasks and then we are given the task, solve it, it goes through two tests and at the end the estimate should be somewhat met.

I am beyond stressed because some of the estimates are simply impossible for me to meet. I still don't know the entire system(because it is quite substantial) so sometimes half the time is spent finding out what i need to do and where and by the time I finish sometimes the estimate is over and there is still testing to be done (and correcting mistakes if they were any).

The second time I have to deal with a similar functionality it all works much faster, but so far I feel like I am just bad at programming.

Is there anything you did when you were just beginning that helped you get over this stage? I get so stressed when I see how little time there is to code that sometimes I can't even focus properly at what I'm doing which makes it even worse.