Yup, I know the feeling all too well. In theory, you are doing everything right, but the time just isn't there, other guys are finding tenths and you are struggling to even match your previous best.
I can use the Vette at the DT as an example, I did a 1:54.2 on the first night of the TT, just driving around and testing some lines and gears, not really pushing too hard, and I thought to myself, nice, 53s are easily doable, I'll do it tomorrow and be done with it. 13 days later, I found myself still on 1:54.2, couldn't get near my time for the life of me, and I literally didn't finish 5 proper laps in the meantime, just errors everywhere, to the point that I start to hate the track, the physics, the car... Following some top ghosts, you do everything as you should, and the ghost just drives away on every single corner exit. Frustrating. But then on the last night, I just said to myself, OK, you can do this, gave it another ~200km and nailed it.
On the other hand - the Mazda TT, I hate the way the car handles and feels from the bottom of my heart, I didn't drive this car in real life, but I can bet my left arm that it doesn't handle this way, not even close. I'm currently at 1:01.185 I think, P333 in the world or something, which is quite far from my usual top 50, top 100, whatever, and I just don't have the desire to even try it, it doesn't click, it's a struggle as soon as I sit in it, and I will not try again for sure, just don't want to torture myself, I'm not improving my driving skills, I'm trying to work around the game flaws. Nope.
But the Alfa TT is a completely different story - I belive quite realistic handling for such a car, predictable, you know what you have to do and it's just a matter of doing all the right things properly in one lap - and you can show your skill, be that achieving silver, gold, top 500, top 100, top 10, whatever.