I trick myself into drawing by deliberately starting with a "bad" idea and aiming the make bad art. What happens for me is can then develop whatever is in front of me and it gets me more in a mindset of valuing the process of drawing/painting over the end result. It always ends up being better than my last sketch/painting too! If I don't aim to make bad art, I end up in an eternal preparation stage for whatever I'm working on - or I spend months only doing study and not creating anything. That's my way of overcoming my perfectionist tendencies which were stopping me from having fun just sketching. I hope this helps :-)

I am new to and just starting browsing. Everything Adam mentioned is great. To add my 2 cents: I have constantly stopped and restarted drawing for many years. I was exactly in your position and was too scared to draw in a sketchbook. Mainly due to feeling inadequate and the thought of, "I would only waste the paper with my lack of skills." My work around was to simply use the cheap, standard printer/computer paper and emotionally feel that they are meant to be used as scrap paper for my practice sessions. A tablet was easier for me as I could just simply erase/delete/trash anything I felt. The best way to overcome your fear, is to just trick your brain as it is meant to be your practice paper (or trash/cheap paper depending on your perspective). Once you start purposefully drawing without the guilt of preserving perfection, you will naturally become comfortable to use whatever media to work on. It will also progress to the point where you will think in terms of your overall progress and begin keeping everything as a guide to look back on. Edit: I forgot to add. This is all psychology and dealing with conflicts within yourself. You are not slow. You are not weak. You are not bad at drawing. You are at a level where you can progress. That is simply it. Work on being confident with your drawings and focus on improvement. Ask anyone and they will be willing to help/guide you.

to break through that stalling due to over thinking, try adding an easy no stakes, standardized 3 minute starting routine. Use a blank sheet of cheap paper you intend to not keep, and warm up by filling that sheet drawing spontaneous lines, from the shoulder, to make natural smooth sweeping arcs, curves, circles, ellipses. After getting feeling of body moving in action by drawing nice linework, then turn to actual sketchbook. Also keep at least one sketchbook entirely to yourself for practicing ideas without pressure of posting or showing to outside world. I hope that helps.