Jake,
First, welcome to the forum. Congratulations on your persistence to go after your goal. With that kind of attitude you cannot fail.
Second, as for the difference between mechanical and automotive engineering, maybe a little history will help. The first established fields of engineering "disciplines" (as they are called) were civil, mechanical, and mining. This makes sense if you think about it. Roads, bridges, dams, mines, canals, were all examples of the detailed application of scientific principles to real world problems. As new technologies came along, engineering disciplines developed specialties that eventually grew into their own established disciplines. For example, electricity, nuclear power, the automobile, aviation, chemicals, petroleum, etc. Now the variety of engineering discipline choices can be mind boggling. But if you understand this basic fact it might help. They all began of branches of one of the basic fields. In my opinion it is best for new students to start in one of the truly basic fields (mechanical, electrical, chemical, civil) and then branch out later when they know more about it. I agree with what you have been told, that automotive engineering and mechanical engineering are so similar as to be almost indistinguishable. For example, an automotive engineer will definitely have to understand all the basic mechanical engineering principles, but a mechanical engineer might not have the detailed courses in internal combustions engines, or fuel injection systems. A good mechanical engineer can get a job designing cars, but a good automotive engineer might not be able to get a job working on space exploration. There's more to this, so read up on it.
You asked about job prospects. I've been doing this for over 30 years and I can tell you if you are good at what you do and willing to be flexible you can always find work. Personally I graduated from NC State in 1973. I've worked for large companies, small companies, my own company, manufacturers, insurance companies, consultants, machine shops. I've worked for manufacturers of electronics. automobiles, automobile suppliers, machine tools, heavy and light transportation equipment, basic steel products, auto glass, lighting, powdered metal, food, printing, and many others. There's a great big world out there and its all fun.
Right now I do what I believe I was born to do. I design special machines for a highly specialized manufacturing process. I get a great deal of satisfaction out of seeing something I designed doing what it is supposed to do and doing it well.
And last but definitely not least: You have demonstrated your desire to move forward and I applaud that. I have been in the position to hire and mentor many young engineers, so take my advice very seriously. Be very aware of and careful about the first impression you make on others. For example, did you notice that it was easy to read my response? A question - if you are presented with two different things to read and one looks easier than the other, which one do you think you will read first? Right, the easy one. My point? Make your writing easier to read. Engineers are professionals. If you want to be taken as a professional you have no choice but to act like one. A professional takes care with his grammar, his spelling, his punctuation, his paragraphs, and his capitalization. Don't do that and you won't be considered a professional. If you want to enter the field then you should approach EVERYTHING you write professionally. You aren't texting your best buds here. My words are strong because I know how important they are. Unfortunately our schools today have chosen to graduate an entire generation of youth with no understanding at all of the importance of the basics of written communication.
And after you graduate I will be on your case about the quality of the appearance of your drawings, but we'll do that later.
Best of luck to you!
Joe