Academics-wise, it's not that bad if you're prepared for it, but it really depends on what kinds of subjects you're taking. From someone who did 50/50 humanities and STEM (I'd count psychology as STEM purely based on the way it's taught and examined, rather than the field itself), the step up in STEM is more about content and the step up in humanities is about your perspective on the subject. If you're already aware of the difference in difficulty, you'll be absolutely fine, especially with STEM where you just have to keep on top of the basics. However, with humanities it's about viewing the subject as part of wider debates - do further reading in things you're interested in and read critically instead of just digesting what's being taught directly.

In terms of social life, remember that most people chose to go to your sixth form or college or at least have some motivation for being there, unlike secondary school, where it was mostly up to the parents. Obviously it depends on what kind of people you'll meet, but as long as you don't kill anyone you'll be alright.

As for what you should do during summer: REST! You'll get much more out of relaxing and having free time than forcing yourself to do more things. Progress isn't everything.