Not convinced by your own knowledge
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.