This doesn't feel like a disaster to me at all. This is an opportunity to grow and learn new things and new ways of doing things. It looks like your environment is very supportive and understanding that it will take some time and effort to get up to speed and are offering mentoring. And businesses often have valid technical or non-technical reasons for using a particular software stack - for example C# developers are likely easier to find than C++ developers which makes hiring easier in the future.

My suggestion is to take some deep breaths reset your perspective. Yes, sometimes it's not fun to push outside of your comfort zone, and it sounds like that is creating a lot of anxiety for you, but you'll do fine. This is very similar to how I got started professionally programming 25 years ago.