I’m guessing that the man in question is also in his 40s. If so, what you see is what you get. He is unlikely to change. You’ve done everything you know how to do to address the issue. As you are finding, you can’t make him want what you want in this area of your life. Most important, he doesn’t seem to share your concern. You did not report that he is upset about his low sex drive or that he is motivated to change it.
Only you can decide if having sex is a non-negotiable for you in a relationship. If so, it is only fair to both of you for you to separate. Talk with your boyfriend about whether he would be willing to continue to support you for a specified time while you regroup. If he can’t or won’t, I hope you have friends and family who will give you some help or maybe let you live with them for a time.
It was probably a mistake to give up so much to be with this man before you understood his limitations. But it’s a mistake, not a disaster. You have many talents. You have qualifications for many kinds of jobs — including teaching but also with any industry that needs multilingual staff or translators.
Explore whether you can get your old job back. If not, and you like where you are living, consider that the Bonn Cologne area is a huge metropolitan area. I suggest you find a job coach or a “head hunter” to help you find the job you deserve. Do an internet search of recruitment services in Germany. It may be that you need to expand your thinking about what you are qualified to do.
I wish you well.