Haha this is the method I used - school of hard knocks and near death. Totally agree with @Charlie193 that if you're second guessing yourself on something, save it for another day. I learned that the hard way. Progression is definitely the key to staying safe and there is no shame in getting off your bike to avoid doing something outside your comfort zone.
To the OP's point though about not knowing what to be afraid of, my advice would be to 1) keep your wheels on the ground until you're able to carefully progress on drops / jumps, 2) keep your speed within the limits of your balance / control of the bike, avoiding scenarios where you're unable to control your speed, such as steep, gnarly descents, and 3) avoid going up steep, technical obstacles that could leave you and your bike tipping over backwards. Also know that being seated in the saddle leaves you very vulnerable in many situations you'll encounter on the trail. The quicker you get comfortable being out of the saddle and balanced between your pedals and handlebars, the less likely you are to get hurt.