same position here!! I took 2 years out and despite having grown in many areas my mental stamina is gone! Now Im a fresher and I only manage 6 hours max. of study per day (including class study!) And I've forgotten how to write essays and stuff too! Remember aswell that you are probably coming from an average sixth form/college to a uni where people are generally more intelligent, especially if you're at a good uni! Also, in seminars its normally the clever people that contribute and the less confident people dont as much, so when you only hear the super clever peoples opinions you presume everyone knows that much, when they probably don't. At least thats what Ive found.
But Im sure it will work out for you, remember how hard A levels seemed at the start?? You obviously did ok in them if you got into uni =)