I normally play tank characters, but in a recent game I was called out by the enemy team for being a bad tank. Frankly, they were right too, I was unable to protect my carries from them. THe problem in this game was that I was cho'gath while my enemy top lane was shen, so he could randomly be in a fight whereas I could not, so not only was my team usually outnumbered in fights, but my mid laner also was completely stomped in lane, only having boots, a RoA and a blasting wand by endgame. Our AD carry was teemo, who was I think AFK for a portion of the game and was also behind, but I'm not positive on that.

The way I usually tank is by getting in the enemy's face throwing out my CC skills at them, and usually we just steamroll right over them. Unfortunately, my team was behind, so we had to back up whenever engaged upon because they generally had baron on them. I tried the same strategy of running into the enemy team hitting them with my skills, but they just walked past me and killed my carries anyway.

(ACTUAL QUESTION HERE)
It all comes down to: what should I do to tank in fights where my team is behind? should I still try and push the enemy back? or should I use my CC to try and focus on protecting my team instead of pushing the enemy away from them? Or was this just a matter of my team being to far behind?

This as more to do with champion selection. You shouldn't pick Cho as your tank if your squishies aren't packing their own CC and escapes as well.

If you're landing your rupture and using your silence correctly, then you're doing pretty much everything you can as a "tank". The rest is about getting in between them at your allies so you can block pathing and skill shots.

There's also the issue of Shen. There's a reason he's usually banned in ranked, and that's in large part because of his ult's ability to turn the tides of a teamfight so decisively. Your team should be playing with Shen's ulti in mind. So if they're engaging 3v3, they should expect it to turn into a 3v4.

And your team being behind never helps. The best you can do there is try to hold a tower against the majority of their team (cho clears minion waves very well and is dangerous to dive against), while your under-fed carries go farm up other lanes. Ward of course to escape ganks.

So, to answer your question directly. It's a bit of everything. Team coordination, champion selection, and bad decision making when it comes to when to initiate teamfights.

If you know you are going up against a Shen, take teleport and get a little AP to push lane hard. Chogath with an early RoA or similar item should push pretty hard. Once you push his first tower, you are free to roam too. And your ganks will be better than his assuming your team doesn't overextend.

You didn't really state your build however. As far as being a tank, there are really two methods. The first is to go for auras and CDR and support your team with your CC. The other is to get a little damage to punish your enemies for ignoring you or pull fire to your tanky ass.

Did you build items like Frozen Heart? With a team that is behind, try going the AP off tank route. Chogath solo top should have plenty of money to build RoA, Abyssal, Sunfire, etc. Then you can punish their squishies for getting out of position or help peel tanky champs off of your squishies with damage AND CC. And you won't have to rely on your behind teammates.

my build was merc treads, RoA, Frozen Heart, FoN and was going for a GA but the game ended by then.

I DID have TP, but I guess I should've used it to countergank more. I never actually know WHEN to roam though, which I suppose is something I could work on. I suppose I was sort of anti-roaming in a sense, as Anivia and Pantheon (their jungle) came up to gank me several times (the only successful one being early on, which I traded for a kill on shen) which took some pressure off of the other lanes I imagine. Those ganks came closer to the mid game though, and by that time the other lanes were behind.

With someone like Cho'gath, your main goal is to be disruptive. You can't reliably initiate with Rupture, but you can certainly cause chaos with it and scream if you land them on the enemy dps. Just save them for an opportune moment and don't waste the cooldowns trying to poke.

Also, you have the ability to scare the **** out of a random squishy target by flash feast ignting them. The psychological effect of seeing half their health evaporate instantly makes a lot of people back off for no reason.