I would advise to download the replays of any of the top 10 drivers in the TTs, watch them to see how they tackle the circuit and then load the ghost....while you won't keep up with the ghost, it is a good incentive. For example, I was really struggling with the GT Academy TT at Apricot Hill.....I know the lines etc but just couldn't get the time for gold..was over a second off....downloaded the Number 1 ghost and, within 3 laps, I had improved my time by 1.5 seconds!. Sometimes it can help to have a faster ghost to attack (though sometimes it can be counter-productive).
Experiment with different lines into different corners.
Deliberately set a very slow lap at the beginning of the TT....then aim to slightly improve it each lap so you are trying to get tenths rather than seconds each lap. Focus on one corner per lap until you nail it.....brake late going into the corner and, if you go off, brake slightly earlier next time then experiment with applying the throttle etc.
Finally, you didn't mention if you use a DS3 or wheel.....if you use a DS3, try experimenting with different button set-ups.
One final suggestion is: Have a look at the WRS in the Online Racing section (this is the Qualifier:
https://www.gtplanet.net/forum/threads/gtp_registry-qualifier-gt6.294799/); this is their current TT:
https://www.gtplanet.net/forum/threads/gtp_wrs-week-17-edk-goes-to-daytona.308630/) I haven't done the WRS for a while but I improved massively by doing their TT's every week....most weeks they use untuned cars, sometimes tunable, a wide variety of cars. There are a lot of fast drivers in it who are happy to offer advice etc. I would say, over about 3 months, I probably improved by about 2 seconds on any lap compared to where I was before.