USA, Canada / How difficult is it to get a visa in Poland for an US citizen? [11]

Greetings,
I'm a US citizen who has traveled to Poland two times. One week in February 2016 and one week in March 2016. During my travel last month, I recently got a new Polish girlfriend and I plan to travel back to Poland for one week in December, and two weeks in May 2017.

Now, I'm writing this because I have recently heard news that EU will require US citizens a visa to visit to Poland:
nytimes.com/2016/04/08/world/europe/european-union-to-consider-requiring-visas-for-us-travelers.html

If the visa passes, I'm changing the schedule of my trip for three weeks in May 2017.
Anyways, how difficult would it be to get a visa? I have friends from Asian countries who say that they have to have letters of invitation in order to go to Poland. They even said that there were times that they had to talk to an immigration lawyer to stay longer.

What would be the questions for a visa in Poland?
Would I need my girlfriend to write me a letter of invitation?
Any advices?