1) the 'rest of your life' speech
I'm not denying that the choices we make now are important but 'the rest of your life' is a bit over-rated in my opinion.
Seriously, the topics I take at GCSE or a-level etc do not form the complete template of my life-I could re-train,take my GCSE's again! I could go on courses! I can go to university multiple times! these could all change variety of jobs that I could be qualified for completely transforming the template of my life, whilst it is true that GCSE's are important I don't believe that my life will go downhill if I get a C.
To reiterate- choosing is important but it only effects the rest of your life if you want it too.
2) It's YOUR choice to what you want.
Parents, especially,can be more than a little persuasive at times and can be rather set on their ideas of what they want you to do with your life. It's counter productive to get mad at them for this as it'll only make them angrier and thus less likely to talk to you about it- you have to remember that they are choosing for you as they think they are giving you the best start in life but what they may not understand is that the 'best start in life' may not be the best type of start for YOU.
For example, when I had to choose what I was taking for GCSE I was 'encouraged' to take a language by many people and so I did. The problem is that I didn't realize that this choice wasn't for me as I have only just realized I am pathetic at learning languages in general (moral: don't let people chose for you as you may end up doing something you don't like without realizing)
So talk to your parents calmly about choices (and if you want to prepare your 'pitch' then make sure you thing of some reasonable responses to counter arguments they may fire) and be patient.
If you enjoy something DO IT because the chances are if you enjoy something then you'll probably produce a better standard of work which leads to a better grade- can't argue with that.
To summarize: chose to do something because YOU want to, don't choose to do something just because your peers tell you to and if they feel passionately about an opinion, talk with them calmly explaining your own view and remember that if you think you've made a mistake then just try your best to make it through.
Timshel-Mumford & Sons
Disclaimer: It is your choice to follow my advice or not but be warned that I'm not responsible for anyone else's choices but my own, this is purely ADVICE.