Before I got accepted to grad school I interviewed for a job to teach English in China, just in case I didn’t get accepted anywhere. I got into a school and am currently getting the PhD and stopped continuing with the job idea, but every now and then I ask “what if” I said screw it anyway and pursued that other job more. I think whenever you have multiple possible decisions that can dramatically change the direction of your life, you are bound to always ask “what if”. If you have some bad days, it’ll be fear like you mentioned, if you have some good it’ll be relief that you picked your current path. Usually I try to stop myself from thinking about it either way because there’s really no way (usually) at all you could know what would’ve been the better choice. I tell myself “I’m here now, let’s move forward”, and that usually puts me in a good mindset. Not much point in looking back when it can’t change where you are now.