Just like any off-flavor, you might really just need to have someone else with experience taste a beer that is obviously flawed, and point it out to you, to help you learn what it tastes like. That’s partially how I learned. Taste descriptors are not exact, but they’re in the right ballpark. So, oxidation might not taste exactly like wet cardboard… however, I do agreee that it is very similar to that.
I don’t get bitterness from oxidation. Quite the opposite, actually. To me, my 4-5 year old oxidized beers (yeah I keep some on hand) taste very much like caramel and maple, with, yeah, a sort of wet cardboardy flavor as well. The caramel and maple is due to the hops falling out completely with age – hops don’t last long like malt flavors do. But the cardboardiness… that’s the oxidation. Age some beers for yourself and find out.
The best thing you can try… purposely age any regular strength homebrewed beer for 3 years. Then, if you can remember how it tasted young and compare to how it tastes with age, you’ll see what’s happened. I’ve done this dozens of times. Not every beer will oxidize, but most will. If they contain a ton of alcohol, it seems to happen about 50% of the time. With lower strength beers, it happens more like 90% of the time. Do this to a whole bunch of different homebrews. Then taste them all side by side. The weird flavor that you detect in common with all very old beers is oxidation. Notice I said to do this with homebrews. Commercial brews tend not to oxidize anywhere near as quickly as homebrews. Some commercial beers will oxidize in a year. Others… it might take friggin 10 years. I have some 5 year old commercial beers that still have no oxidation, and I’m sure this is because they work really hard to eliminate any oxygen in their bottling process.
Okay… so, back to the OP. You might be experiencing astringency, which many people perceive as bitterness. Astringency is different from bitterness in that it is actually a feeling you get in your mouth, as opposed to an actual taste detected by your taste buds or nasal passages. Astringency has a tendency of drying out your mouth, as if you had filled your mouth with cotton, or chewed on a grape skin for way too long. Try this with grapes sometime… start to chew up a couple of grapes like you normally would, but then just suck all the juice out of them and save the skins (in your mouth), then chew and chew and chew on those grape skins, and notice what happens… that’s astringency. Like your mouth is full of cotton. Is that what you’re getting out of your beer??
If not… I might really just need to taste the beer myself to know what’s going on. Could be too much brewing salts, could be too much bittering hops, could be a couple dozen other things.