My greatest obstacle is myself. It's not even in the category of fear... it's that my first assumption is always that I "can't" do something; that I need it simplified or made smaller, or that my default is weakness. This is incredibly destructive and I suspected it for much of my life but only really started seeing it in sharp definition when I started parkour. Arguably it makes me the safest traceuse on the planet

but it also means I have some major mental baggage with stuff and this slows my progress.
I first *really* started to see it last week, actually, when I was training with Chad. We were doing deadlifts with kettlebells and I chose these little weights, like 12kg. Chad was all, "You can do more than that. Here, take these," they looked huge to me, I said, "Those are too heavy for me," and he asked, "How do you know? Did you even try them out?" I picked them up and they were heavy, but Chad made me do the deadlifts with them. I did fine! They were a challenge but they were definitely doable. Afterwards I realized I had deadlifted 106 lbs. for all the reps of the day. That was the first moment I realized that maybe I really was stronger than I thought.
This has really changed my thinking--for instance when a recent workout called for HSPUs, instead of thinking automatically, "Oh, I should work up to these," and assuming they were beyond me, and going for the scaled-down exercise, I thought, "Hey, I should just try one and see if I can do them." I ended up doing two! They were hard, and kinda wobbly, but I did them. And I discovered I could do something way beyond where I was assuming I could.
I don't know where I picked up this habit of putting limits on myself, but for me, the obstacles I put up in my mind and heart against my own progress are the toughest to overcome. Rationally, it's a really stupid thing. I mean, why wouldn't I believe in myself and encourage myself? It's not like I have self-esteem problems, but I just assume that I am this weak little thing all the time, way deep down inside; which is counter to everything I stand for in terms of female equity, etc. Heh--the men in my life have all been a thousand times better at believing in my capabilities than I have. Isn't that silly?