(1) The whole situation is too fickle to control, so I just let it go due to/because of my anxiety. (= so I just let it go to not worsen my anxiety). (2) My boss is so fickle! One minute she chatted with me in a friendly way, but the next minute she lost her temper. (3) The world economy is so fickle. You build a start-up at your own risk. :)

Very close...great job using this word and applying it to the right context. Here are the few adjustments we should make. (1) The whole situation is too fickle to control, so I just need to let go of my anxiety (or I must let go of my anxiety). (2) My boss is so fickle! One minute she chatted (or was chatting) with me in a friendly way/manner, and the next minute she lost her temper. (3) The world's economy is so fickle. You need to take the risk of building a start-up. I made a few adjustments for you. If you have more questions, just let me know. Thanks. :)