While I did not study Mechanical Engineering, I did major in Industrial and Systems Engineering at Georgia Tech, which has some similarities. Engineering is tough, but like with anything, you can certainly succeed with studying and hard work! Some classes are more challenging than others, and that likely depends on the person and what you like vs. don't like as much. Forming study groups, and meeting with teachers or assistants during office hours can be a huge help with you come across a tricky topic.
I have a few suggestions for avoiding and handling stress. First off, even the easiest major could be stressful if you don't prepare yourself properly! So don't let that deter you from ME if that's your dream. If you stay on top of your classwork, reading, homework, etc. you may have less stress when a pop quiz, test or exam comes along. Secondly, it's great to have other outlets such as sports or something creative to burn some energy and turn-to when you need to give your brain a break. I'm sure you've been told for years about exercising and eating healthy :) It really does make all the difference!
In terms of learning more about Mechanical Engineering and Auto Mechanics, see if you can meet some people with that career. Ask for an information interview or buy them a coffee to talk. Perhaps look at some colleges nearby that may have info sessions, certifications, or classes in these areas. They should be able to put you in touch with current students and teachers.
I hope this helps. Best of luck James!