Jimulacrum
Full House
This is on point.To add the detail, it's either or, if you bet this river you are hoping he will make a considered laydown with a hand he 3-bet pre and led twice, or he will pay off a weaker hand (which that range just got narrower with the bottom flop card pairing.) There is no world where villain is calling with 6x but making a considered laydown with 99.
We all fall into this trap from time to time where we justify illogical actions, and we just compound mistakes street after street. I probably would have been tempted to call the flop, but I landed on a clear fold after typing out my analysis. That's why strategy threads are useful. Forces you to take the time to analyze situations before they happen again.
Betting the river in this spot only makes sense, as with most of the actions in this hand, if you know Villain and have specific information that makes it a good play. Like, say, you know he will often three-bet and then double-barrel with unimproved ace-high, and he is also capable of snapping off river bets with ace-high (but will check back if checked to). Then you can bet 88 for a bit of value.
Outside of a situation where you have this information, the best way to describe your line is "got sticky with a pocket pair and potted river for no clear purpose." I've had many players over the years take lines against me like the one you took here. It's something I make a written note about if I'm playing online. Getting overly attached to pocket pairs is a massive leak.
Moreover, the river is a different animal compared to the flop and turn. On the earlier rounds, you can have a legit reason to bet out or raise with a marginal hand like 88, to protect your hand and force weaker hands to pay when you're ahead. On the river, even if you believe you're good, Villain will almost never call with a worse hand. If you believe you're losing and it's a bluff, there's not much you can bluff out. Some middle pairs like 99 may fold, but even that is a stretch because your line is so suspicious. You're repping a 5 or better, essentially, but your actions aren't really consistent with that.
In fact, your actions look like precisely what you have: a marginal hand that you hate the idea of folding but aren't confident is ahead. Even if Villain didn't have a 5 here, and he had one of the other hands folks thought he would (overpairs, mainly), that river bet is little more than a gift to him. If you called his jam on the end, that's even more unfortunate.
I recommended folding the flop and the turn, and I stand by that, but calling on those two rounds was nowhere near as leaky a play as betting out (and calling, if true) that river.