I like to use liquid liner and a felt tip is easiest for beginners. It tends to look a lot smoother and cleaner than pencil (if you know how to do it right;). Now I know this probably isn't what you want to hear but it's the truth- it really just takes a LOT of practice. I was exactly the same way and I just did it everyday until I got it right and figured out the style that worked best for me. Some tips I could give you is to keep the side/knuckle of ur thumb balanced on your temple while starting in the corner of the lid and making your way towards the end using short lines and connecting them. You really just have to get a feel for it but you'll get it eventually dont worry:)
I basically start off with drawing a straight line (the base of the wing) and fill in the rest. I have eyelid creases (idk if everyone has them) but i basically follow those to fill it in. Make sure to let your eyes dry before blinking extensively. My favorite type of eyeliner is a liquid pen.
I don't do top eyeliner very often which I believe is what you are speaking of. For the bottom I just start on my waterline and slowly move down until it's the right thickness. For the top I pull my eyelid out so it's flat and start brushing in your eyelashes and go up a little bit at a time in small strokes. After you have the basic line go over it again to smooth it out
Start with an ultra thin one or a precision liner. Use the liquid pencil one. Try using Ulta's precision liner (or similar to that). That's how I learned. I never had the "too thick" problem because of those liners. Then when I've mastered the cat eye or just a regular line, I started using other liners, thicker ones.