I did really well at school except for the final exams where I got terrible grades. Thoughout the years, my grades were above average but I got terrible grades in my finals(which count for 50% in my country) and that caused me to get a GPA of 2.4. Should my math teacher talk about how/why i got bad grades in my finals? It's quite clear that I almost failed because of those grades but it's also quite clear that I was an A student before those grades. If he doesn't talk about my math abilities(or disabilities) then what should he talk about? Please help me, I need to send this letter next week
Your advisor might be limited by local law about what s/he may say about your grades. But they can probably say that your grades don't completely reflect your abilities and then comment on those abilities. But, IMO, a too-detailed letter won't serve you as well as one that is more general and speaks about your abilities. If they focus on the positive then it should serve you better, I think.
However, if you have difficulty in exams you should find a way to work on that. Perhaps it is just ineffective studying. Cramming is a poor way to learn, if that is it. Taking and reviewing better notes throughout the course is a much better way to learn.
When students ask me for a recommendation letter, I often ask them to send me a resume and a bullet-point list of their academic and other accomplishments. For students who have been affected adversely by reasons of finance, family, or other issues, I ask them to include those as well. Assuming this is the case for your final exam performance, you should note this as well. If you have not sent over this material as a "refresher" for your professor, you should feel welcome to do so.
As a matter of etiquette, please do not request that the professor exclude what you might find as a matter of fault. It makes you human. Additionally, professors are not required to write recommendation letters for students, so please be nice to them. Otherwise, you may find yourself being refused a rec letter.
I assume you are applying for graduate school? The graduate schools you are applying to have access to your transcripts, so they will see your grades regardless of what your professor writes. Indeed, if you are applying to graduate school, it is likely that you will be taking many exams during the course of your studies. Be prepared that you will need to foster the skills to pass those exams as well.