I work at ABC corporation and this is my first job. Over the past two years, I have completely redesigned ABC's corporation's DevOps system using programming language X in which I am proficient. The previous system was "spaghetti," costing the company hours in debugging efforts, etc. The new system follows ISO/IEC 9126 standards and has accelerated company development efforts.
Because of this, the company has deemed me the "best X developer" (which I find funny... because all I do is literally just write the simplest possible code whilst maintaining consistency. I also have a bit of a soft spot for unit testing and documentation) and have asked me to take over a high stakes project.
The high stakes project (let's call it System X) is written in programming language X and it's... bad. The project consists of a single God class consisting of >60 member functions. To top it off, each member function makes calls to other member functions and some functions are 100+ lines long.
It gets worse. System X synchronizes events between systems written in programming language Y, in which I am not proficient. Let's call this System Y. System Y is much more complex than System X, completely undocumented, and no one has time to "teach me" how it works.
A mistake in the System X/Y pair could cost ABC corporation significantly in the form of litigation.
ABC corporation is upset that I take long to fix client issues. ABC corporation assumes that I just "magically" know everything about System X/Y because I am proficient in programming language X, whilst neglecting the fact that System X is poorly written and the fact that I have never received training with regards to how System Y works.
I went in expecting to deal with legacy code... I think this is a fair expectation... what I didn't expect is to be expected to fix issues with legacy system X, which controls undocumented system Y, which can cost the company severely if I were to make a mistake, all with extremely minimal training.
I'm starting to really burn out because of this...
I have already communicated this to my manager and he more or less kind of has a "I don't have time... sorry" attitude. How can I communicate to my manager that this is actually quite serious?
I'm also seriously concerned about my long term career prospects if I were to make a serious mistake this early in my career.