I have to provide estimate of a story, to the point where I have to tell how much I will be spending for each sub-story.. although I have given them those with buffer since I am new, I notice a pattern with me which is that I usually need some help which I get either at the end of the day or the next day. This can influence the estimate ofcourse but for some reason my manager wants a very aggresive dates, and it's challenging for me.
In addition to that, I am not sure what I will say in order to justify if I can't meet the deadline. This process has sucked the life out of the work for me. I remember giving estimates in my past companies but they were just estimates, and when I shared my side of the story people usually understood my challenges this is not happening here..
Sorry to hear about how demotivating the process has been for you. Here are my thoughts:
Adding on to this, it seems like your manager is fundamentally unsupportive. If they're not accommodating of the above points, no matter how clear and polite you are, I highly recommend looking to switch teams, find someone else as your main point of leadership/mentorship like a supportive tech lead, or exit the company. Here's our in-depth session about finding the best team/company for you.
A broader question to bring up is that the actual planning/estimation should be not be too expensive. Most of an engineers' time should be spent on execution, not the meta-work like giving updates or estimates.
Are other people on the team also feeling burdened by the stress + time it takes to provide an estimate? If so, that's something to (separately) bring up with your manager, on how to improve that process.