Not convinced by your own knowledge I'm 26 and an Bachelor-Student in the Electrical Engineering field.
My interests are Control-Theory and Robotics.
My Grade are good (maybe i get an 1.x Grade for the Bachelors Degree).
My Control-Theory Prof is very convinced of my knowledge and gave me the advice to go to an University.

Im currently studying at an Fachhochschule and want to go to an University for an Master Degree in my interest field.

The Problem is, that Im not very conviced about myself. Im thinking that I have to learn everything (math, physics) from Scratch to be prepared for the Study there.
The Problem is not that Im not ready to invest a lot of time for it (I am!), but I think that i should learn the Mathematics from Scratch again for getting an feeling about whats happening (control-theory is very mathematic).

I can calculate a lot, but developing own Ideas and get behind the Ideas of other People is at some point very difficult and ends often in accepting things. I want to have an intuitive feeling for whats going on, and for that I think I should repeat the mathematics (Analysis, Linear Algebra, Integraltransformation) and start to dive in Functional-Analysis.

At this University the Master-Course starts twice a year, April and October.
I have emailed the Faculty and being told, that I have to complete the Math III Exam with is part of there Bachelors-Degree Course, and maybe another exam which i dont know (maybe theoretical electrical engineering).

My idea was, to spend the first Semester to get skilled in mathematics and consist the 2 Exams, and maybe If the time allows, to finish some other Courses which are held in the first Master-Semester.

My problem is, that Im 26 and I think Im very old. Im working since 2016 as an Working-Student at an Global Player in the Research&amp;Development of Photovoltaic Systems.
If I see other students which maybe starts with me.. they are 21,20..
But I think Im doing right, if I want to employ in that Field, I have to learn. Im living only once, and If that is my dream, I have to forget my age.

What do you think about my Plan?
Sorry if this is very confusing to you.

Have a nice day!
There is no reason to panic. 

A 1.x grade in Bachelor is good. Master's programs in Germany in Electrical Engineering tend to be less mathematically heavy that Bachelor's. They also tend to leave you enough free time, which some students use to work for money and you can use it to revise the basics in Mathematics and Physics.

You do not need to start from scratch. If you got good grades in those courses, you will be able to pick up the content quickly again.

It is normal not to fully understand every single idea and have to "accept" some things at the beginning. Intuition comes with time. 

26 is not too old. Many students (especially international students) get a job for a few years between Bachelor's and Master's. Starting a Master with 26 is not the most common thing but it is not rare.

You seem to be worried about the switch from Fachhochschule to university. Don't be. Mathematics and Physics are the same everywhere. From what you write, you seem to believe you performed well in your Bachelor's. Your professor also thinks you are ready: listen to them.