Catrin, I can feel your frustration. You will regain your fitness. It doesn't matter, really. As long as you are on your bike, no matter how slowly you go,or how long, you will be out there. It will just be another challenge.

In 2007 I rode my bike 4 times between September 30 and December 7th. Without going into great detail, I was sick, and having anxiety issues, too. Yet, I was able to walk/hike and do yoga. When December came, I got out the x country skis and was so happy to see the snow. I rode the trainer, too. And when March came, I got back on the bike outside. I may have been slower, but I still rode. I just started where I was and tried to enjoy it more, rather than focus on my fitness.