It’s impossible for me to confirm a diagnosis on the basis of a letter. What’s important is that you are taking responsibility for trying to figure out why you are having difficulty in school and in life. That’s an unusually mature thing for someone who is 16 to do. I applaud your efforts.
Ideally, you would get some testing to confirm your guess. I don’t know enough about how the schools in your country are organized to know who to refer you to. In the U.S., it would be the guidance counselor. Perhaps there is some equivelant role where you live. If not, you could see if your doctor or a favorite teacher could tell you where to get testing and how it is paid for.
If it is impossible to get tested, you can still do a great deal to help yourself. A diagnosis is only useful if it is used to plan treatment. Let’s say you are right. What would you do then to compensate for the disorder?
I suggest you research it carefully and narrow down your symptoms to a few that are the most troubling. Then find out what is done to support students who have to manage the symptoms in order to be successful. It may be that you need a tutor. Or perhaps you need extra time to do your homework and exams. Maybe you need to sit where there are fewer distractions. I’m sure you will find some helpful advice if you continue your use of the Internet. You might also try to find an online support group to give you advice and support.
People do not die from Discalculia. They are challenged by it. You have shown great maturity in facing that you have a problem. I have confidence you will continue to use your intelligence and determination to figure out how best to work around it.
I wish you well.