The problems from the workbooks I am doing tests from (ex. KAPLAN, BARRONS, PRINCETON REVIEW) are all much harder than the practice problems on the college board website. Does anyone know if the difficulty of the test is more similar to the ones on the college board website or practice books?
Since the test covers a wide variety of topics, it is hard to say. For example, there could be a few questions on the real test about hormones. If you don’t know your hormone facts, then that section will be difficult for you. If you know them, then it’s easy points. I’d say the collegeboard website sample is easier since it targets very general information. Barrons is a little bit of overkill, but it can definitely be what you need for a perfect score. However, there are a tonnnn of details in Barron’s that aren’t tested, and that can be time consuming to study. PR is good overall (similar difficulty) for content review if you don’t have too much time. 
PR has much more info than Barrons but it isn’t superfluous. There are some things that could be on the test that Barrons doesn’t cover at all (happened to me) but you will be better prepared for everything else. PR was useful for explaining certain topics in a clearer and more colloquial manner, but overall Barrons will be your best option (in my opinion). Don’t even bother with Kaplan. A lot of the info is wrong and the practice tests are hard but not because they are at a high level, they are just poorly constructed.