Yeah the difficulty increases globally and not per Girl.

The best way to get more points during dates is to upgrade your "skill" I would suggest starting with Passion since that helps you the most I feel and it helps with every girl, as opposed to the other upgrades which might be negated by one or more girls disliking that trait.

Also earning Nectar through conversations isn't a bad Idea since you can even earn diamonds from doing that (1 diamond a day if you find out 3 infos about any girls = 15 conversations answered right)