Help!! I just need some advice right now...really stressed out?

okay so I am sopre in high school and I am having a horrible time this year...I am barely ping my chemistry and algebra 2 cl but right now we get to choose what cles we want to take next year..I really wanted to take AP environmental science but I am not qualified to take those cles, my choices are chemistry, physics, chemistry AP, biology. For math it says I am qualified to move on to math ysis but I need to p the next semester with a B(same goes for chem) currently I have a D+ in both cles...in the end if I do not pass I will not get credits and I will repeat and I might have to repeat senior year...I am reaaaaaaaaaaally stressed out and luckily both my chem and math teacher have said they are willing to help me out but the thing is that I really don't understand the material in that class...and i am afraid I won't pass....also for guitar next year I wnat to take guitar ensemble(pronounceed awn sawn ble, sorry i cannot spell) but I heard you have to audition and I have not played the guitar in a while but i still remember chords and some notes....should I try out? sorry I know this is a lot to read...thank you to anyone who read this.