Why is it that when using good form generally the movement becomes harder? Especially in the bench, before when I wasn't pulling my shoulder blades back I hit 190 for 4 but with my shoulder blades pulled back 170 for 5 is pretty difficult. What's the reason for this, I would think it would be the other way around since with proper form it has such a short ROM
Good form is OPTIMAL for lifting maximal weights. Your form may breakdown some when it comes time to hit that real 1RM, but if you trained with good form and can mostly maintain it at 1RM level you're more likely to get the lift.
Your -PERSONAL- form, the form you train with, is likely where you are strongest. It may not be optimal, but it's the habit you've built and what you've made your body accustomed to.
In other words, if train with poor form, you will likely be stronger with poor form. However, having good form will enable more future gains.
Take a few steps back so you can go further.
I also think, unless you are one of the gifted and talented athletes or lifters out there, form is like one of those Buddhist paradoxes. "Admitting you know nothing is the secret to knowledge."
You may think you got your form down and feel confident and then 6 months later have an epiphany and start reworking shit. If you're lucky to have a good coach or train in an awesome gym you can probably fix this stuff early on, but if you pretty much train alone and are not a natural athlete, expect working on form to be a very long and on going process.
Simple, you are trying to bench with muscles you 'don't have'. While it may suck now, in the long run you will reap the benefits of doing things properly.