Just remember that qualifications don't really prove intelligence... They're just a snapshot of how good you are at answering exam-style questions of the moment of your exam. If you need the grades to get in somewhere, work for them, but don't waste your energy getting good grades for nothing - some people have mental breakdowns over them. Just look at the big picture and remember that there are better things in life than a letter. If it helps you get somewhere - great, but if it's just for the letter, then what's the point? Don't let people make you feel because you're "clever" at GCSE you have to get A*s everywhere. It's a very very rough measure of intelligence.