Hello,
I guess it has to do with practice. On such a tight and difficult track, you have to know perfectly each curve. So maybe it has to do with some experience of the track ?
Monaco was the best FGT race for me by far, very exciting, a nice challenge and the feeling of a deserved victory at the end.
Some setting up with the car will help too, you should be able to find that on this forum. Remember to tweak the downforce to max, shorten the gear ratios.
Both Massenet and casino curves (left and right corner after the top of the hill, at the beginning of the lap) are challenging, you have to be careful there or you will end up in the barriers, as the AI does sometimes.
Try to avoid breaking too late, use that feature only for overtaking ; otherwise, you have to put the emphasis on the exit of the curves, try to be as smooth as possible.
You'll find out that with confidence built in, you'll be able to shave off many tens of a seconds, or even more.
Hope it helps a bit,
Best regards,
Snypa