I can attest to the fact that Respawn's skill assessment tool is kinda all over the place. I've seen mine swing 30% of the way across the curve in a single day, and then swing back again the next. I'm not saying that either placement for me is completely wrong, but I am saying that the algorithm must necessarily have a very narrow definition of "skill" in order for mine to swing that far that fast. And the truth is probably in the middle, as it so often is.
In your case you have to do the impossible, which is to separate your subject feeling of inadequacy from your actual performance. Subjectively there is no way the game can put any normal human in diamond/master lobbies and have them come out feeling like they own the game. On the other hand, look carefully at your career k/d, win rate, average damage/match, and even average placement (not just the numbers from the current season). If any of those numbers are maintaining above average, or even at average, then the lobbies you're in are probably the ones you ought to be in even if they feel terrible.