Mechanical engineering is not easy to teach yourself. If you had the right books and access to a metalworking shop or something like that you can, but otherwise I doubt it. I studied in high school, what I remember the steps of mechanical engineering were:
1st: 2d mechanical drawings. Learning the projections, cuts, mechanical engineering patterns and symbols(for example the stripe pattern means you're seeing the inside of a virtually cut part).
2nd: Then after that a few types of metalworking of course with steel, if you can work with steel, you can work in 99% of metalworking shops/factories in the world. Lathing, milling, welding, casting, sheetmetal bending, cutting, punching, forming are mainly used to manufacture most things. If you can see those in person, the whole production, know how they work, even try manufacturing something, that will be enough for this step.
3rd: Next is, well we call it мeтaлoзнaниe(metal knowledge). This part is the part to study without practice. It's also the most important for serious engineers. You study metals here. It's important to know steel and other metals. I used to have a whole book, that was basically a legend of steel letters that indicate what the steel is. There's tool steel, medical steel, stainless, hard, soft, etc. All are different and are made for different purposes and by adding different metals to add different properties to the alloy. Another part of this step is material resistance, it's a subject about the resistance of structures, vibrations, dynamics. Between those two, there are few more subjects, can't exactly remember right now. Learning ISO, DIN or whatever your country's standart is really important. You need to know what kind of raw steel is sold where your job is, I worked as a mech engineer and at least in my job it was part of the job to know available raw steel.