When is it impostor syndrome? When should you give up?

I've seen various posts detail advice for dealing with impostor syndrome. I know that I have impostor syndrome, but what I struggle with most is knowing when it's merited. In EA, it's really hard to know if you're doing enough, especially when you could almost certainly be doing something better at any given time. 

One way that I try to check impostor syndrome with myself is by asking "Do I feel like I fully tried to achieve this goal and failed? Or did I mostly shy away from it due to fear of failure and, because I didn't fully try, failed?" But, this leaves ambiguous cases. Sometimes I've kind of tried things multiple times, but without full conviction, and am unsure if I'll ever be able to get over my fear enough actually fully try. In this case, I find it hard to differentiate between being an impostor and being a bad fit because I have too big of a mental block. 

I think it would be useful to hear people's general advice on figuring out when something is impostor syndrome vs when you have evidence that you aren't skilled/a good enough fit for a task. I'd be especially interested in hearing from people who have either:
- felt like they weren't achieving enough in a role, quit that role, and in retrospect still agree that quitting let someone more capable take on their initial role
- felt like they had a large mental block that didn't let them approach some type of work they found important, but were able to get over that block and now see their earlier block as impostor syndrome