This is a tricky one to answer, so I'm going to start with my standard first question - why do you want to study Psychology?
If you're planning to become a qualified Psychologist then you'll need to study Psychology, regardless of whether its your best subject or not and you'll need to decide if that is right for you. Bear in mind, if you're planning to return to the USA that a UK degree probably won't automatically grant you access to the further qualification routes (each country has its own accrediting body).
My guess from your post is that you're not specifically looking to become a qualified Psychologist and are just looking to complete a degree. It really depends why you are thinking of choosing Psychology as that degree? Are you anticipating that it will assist you in a particular career path? I can't really speak for a) the difficulty of a Psychology degree nowadays, though on the whole, they are designed for people who have never studied Psychology before or b) the difference in a typical USA degree/Psychology degree and one in the UK and whether they are easier or harder.
I think that on the whole, provided you apply yourself to the degree, that most people do perfectly well with Psychology - getting a 2:1 or a 1st.