I struggle with some of these same things. I get obsessed with an activity/hobby and it will take up most of my day and night but I cant let it go. Most recently it was redecorating my house, I spent hours at work looking online, shopping, designing, obsessing over couches, paint colors, curtains, etc. It was almost panic inducing trying to make decisions. Then bam, before I was even finished I didn't even want to look at it anymore! I still haven't finished the ceiling work. I tend to get like that as a way to occupy my brain when I am spiraling into depression. I have done this with sewing, crocheting, other crafts. Like you, mine will last a few weeks to months, but not longer.

I am also an admin assistant, and have trouble moving further in my career cause I don't know what I want to do, (I tried the legal secretary route too!) but because I know I do exactly like you say, I am really afraid to start a new career cause what if I change my mind! I know exactly how you feel!! But, as long as my job is super busy and has me running around, I am happy doing it.

I also am a very black and white thinker, but don't think that it is related to the obsession part.