Hmmm... well I'd say that if after 36 levels you are not enjoying your toon, you should probably consider something else. I'm not sure where to start to explain why a controller is useful for a team, especially in the mid/late game. Sometimes for me the question is "why do we need a tank?" 
. Of course each AT will bring something to a team and your "role" is to make sure your team takes less damage (and does more). Well with Epics, you can do some nice damage yourself, but that's a different story. You're not really a substitute for a tank (and vice-versa). But what is it exactly that you don't seem to enjoy? Gravity is a somewhat tricky set to play since you don't have many "hard" controls early on. And FF is really a secondary for teaming, but I think it really boils down to what you enjoy playing, so maybe it's just not a controller 
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Slot force bolt with 3 recharge, stick the power on auto and target through the tank - I guarantee you'll feel useful to the team again (although possibly your stay in the team will be a short one).
Seriously though, I've been in your situation from both sides of the fence. When I used to play my tank a lot I always felt a lot less useful to the team when there was a good controller on the team. I was lucky with my controller as he's Illusion so having a tank on the team didn't impact me a lot as I wasn't trying to lock down mobs like other types of controllers might. If my control powers weren't required I'd just focus on my secondary (kinetics) and play as a sometimes controlling defender-lite.
The only advice I can give you is adapt your playstyle to the team make up. If you're not needed for total mob lockdown put a little more focus on using your secondary or doing a little strategically placed damage. Being flexible with your playstyle is one of the keys to enjoying the game in my opinion.
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Controllers actually arent useless at all, they are great for tankers to have around, whilst foes are controlled they are less damaging to the team so you can more easily up your setting if ya want.
All you need to do is find out where and when your powers work best for all and stick at it, aoe control effects around whats tightly herded around the tanker and single target any strays the tanker hasnt seen to taunt and may pose a threat to someone.
When i have played my controller mobs die too fast to even need a duration in the holds but i dont think that i am not needed as the fights done without me are actually even harder. Its much nicer to have a tanker herd, then the mob controlled so everyone can go full out damage with pure freedom and no fear.
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I actually posted a small paragraph on this in one of my troller guides (don't bother looking, it wasn't that much)...
Basically you both provide crowd control. If you're feeling useless then you're either teamed with a good tank (unlikely, but my congratulations if that is the case) or there's not enough people on team to warrant the amount of crowd control you both provide - in which case either up the difficulty or add extra damage dealers (to increase to spawn size and do something useful with all that control you are supplying).
I'd take it as a sign you were doing something right.
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my main is a grav/ff controller, but whenever theres a decent tank on the team i start feeling pretty useless... can anyone point out to me the advantages of controllers (apart from the secondary buffs) before i go roll myself a tank?