I don't exactly agree with you here. Your line seems fine against a reg, but villain here is a fish at NL2. Unless OP has any specific ready on this player, I would assume that a fish who check-raises the turn is never not going to showdown.

For me, the mistake is on the turn: Don't try to bluff somebody who cannot be bluffed. I would assume that more often than not, he has at least a pair or better that he won't let go on the 8 of diamond.

Yeah I'm in agreement here, in poker you bluff to incentivise someone to call as otherwise they have no reason to unless they beat your value however some people do not need to know that you can bluff in order to call you down. Also the incentives work the other way, people need to call you down at an appropriate rate to incentivise you to keep bluffing, if they call too much you stop and if they call too little you overbluff. There's a lot of money to be made in identifying the players who are sensitive to these incentives and you need some degree of balance against and the ones who aren't, as a coach once said to me equilibriums are fragile and we only maintain them when we're not sure which play is more profitable and we know our villain won't let us get away with being obviously exploitable.