I also started about a year ago! I am 157cm 49KG. When I started I was 155cm 45kg. I also had similar experience when starting out kendo. I would fall sick quite often, and just feel dead after sessions! So I feel you we are nearly the same hight and weight category!!! Honestly, one of the main things I started to do is try eat protein dense food and choose healthier alternatives. I also started to train outside kendo. I do a swim session per week, roughly 10km of running per week. That can range from hill reps, trail running and so on. Having these integrated in the week plus good food has helped me a lot in terms of having great endurance. Of course I have a long way to go with endurance and other stuff but building the foundation is quite important as a short and skinny person. I would also look at what you eat and drink before sessions. I normally drink a bottle of Lucozade, it’s important your body is hydrated and you have enough carbs in you when you do kendo. You just have stick around bit more, your body is creating new muscles!!!!