You have a limited supply of fucks to give. Mark Manson, The subtle art of not giving a fuck

Choose your battles wisely. You have to focus. Thomas Frank has a post on this.

My recommendation is that grades are important until they are enough, you have to excel only in the courses that truly matter to you.

Also Jaime Balmes has a quote on this in his essay about criteria.

http://www.clerus.org/bibliaclerusonline/pt/cbi.htm