Fresher in Tech got assigned to a Cybersecurity project in place of Senior SWE.

So I am a fresher who joined this decent company where they pay me a decent salary. I got allotted to a project in place of an Senior Swe. It's been 2 months and I have been given tickets regarding features requests . The work is not easy at all. The team consists of 5 people where there are only 2 developers me and my manager. He is mostly busy so he just provides me an overview about the code base which is 30 years old . The languages are C, C++ and C#. I have been working on this feature request for the past 3 weeks and the pressure is immense. I am feeling so terrible that I am not able to deliver. I just don't like this feeling. I try and try and try but some things are just way above my expertise. I have always been good at coding but to contribute to a real world project is really tough. It's also quite frustrating as at every step you have to deal with multiple problems. My manager is a good guy overall but today he told me that I really need to amp up my game. And deliver. I just don't like to be mediocre but I am just stuck at this point. Am I overthinking? Or me feeling this way is normal. I just want suggestions from different people of this sub!!