------------------------------------ Do I always need to have good marks? ------------------------------------ As long as you are genuinely interested and love what you are doing, things are well, even when you occasionally miss the marks that you set yourself. But if the latter happens it is a sign that you need to rethink your habits. In science, I used to be good at school - things were so easy that I hardly needed to prepare. But at university, things got harder and at some stage I needed to learn (through bad experience - being too careless in the beginning of a course and suddenly losing track of what's going on) that we all advance easily until we are at the level - different for each one of us - where further growth means consistent and serious work. Then we need to accept that and work! And to compete with the best, it takes much more than 40 hours a week.... Now I am professor and head of a large research group. You need to think about quitting math or physics only if you can no longer muster interest, or if the very best (!) of your efforts don't make you succeed.