1st Year Uni student here!
Don't let people tell you that success is impossible during A levels. They're hard, but hard work also pays off. Go in expecting to work hard, don't pressure yourself to get good grades immediately, but do keep trying and keep believing that you're capable of progressing more. You do have time to have a social life, you just have to decide every day what your priorities are, and to look after yourself by keeping on top of work (where you can!) and having time off. Something that can help with understanding lessons in content-heavy courses such as science and/or history is reading the relevant chapter of the textbook (and even making notes if you're feeling fancy) before the lesson, and using the lesson to clear up anything that doesn't make sense, but again, don't worry if you don't have time to do that all the time! You'll honestly be fine at sixth form, I preferred it in every respect to the lower school, and the time really does fly by, even if it's difficult in the moment!