Another American here. I vastly prefer SI units when it comes to small units (g, mL, or mm) and temperature, but I can't really picture what anything larger (L, kg, km). I can visualize an inch, but not a cm, or a gallon, but not a liter.
HolyExLxF lol, I was the same, I just remembered the following until I got it: 2.2 pounds per kilogram 2.54 cm per inch 3.75 liter per gallon (I guess it also helps that I live in Germany because of a military move)
HolyExLxF don't give up on it, just use it as often as you can and you'll get the hang of it. Since moving to Germany I have caught myself, every once and awhile, using meters and kilograms without thinking about it.