You should have folded or raised 3x on the flop for sure.
If you just call on the flop, you are most likely folding or hero-calling on turn and river. The chances of improving your hand are not very likely. You need 9 or backdoor straight. You called an unknown opponent 3 times with middle pair. It's a good board for nines, sure. But you have zero information on your opponent.
This is just really bad poker play, sorry.
By raising the flop you'll either have a much cheaper showdown, or a much easier fold on any of the next two streets. And the chances of a successful raise-all-in on the river become greater after raising the flop.
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