Spiraling about my major

Hi everyone!! i am currently freaking out about the quality of my education and my current knowledge. I am a 3rd year marine and coastal science major with an emphasis in biology. I feel like my last three years has just been doing prerequisites. I feel like I know nothing about my own major. Most of my focus area classes are either being discontinued or are only offered every other year, and pretty much only in spring quarter.
Some may ask, what about your advisor? I don’t have one. I don’t have an advisor. She got moved to another department and left mine with nothing. And even still. she and I mapped out my junior year before she left and she failed to mention a pretty important class that was only offered winter quarter every other year and i missed my only chance to take it. I understand that I also have a responsibility to make sure im taking the right classes, but I can’t remember everything. This was her job and I feel like I have been failed.
I’m not sure what to do at this point, how to mitigate this feeling. I feel like I should have gone to another university. I feel like an idiot for choosing this one. I don’t know if anyone else feels/ have felt the same; if you have, any advice on how to get past this would be much appreciated