Dear Roshi I have been struggling with an overwhelming sense of sadness during zazen. I seem to sink deeper into that feeling when I'm sitting and I feel stuck. How can I deal with that the best?

The human joy and sadness are the same, they are emotions. They appear according to the circumstances and are different expressions.

If you do zazen and face deeply within, these emotions appear more clearly. When you are busy in the outside, it is not so easy to notice them. There are people who are more emotional and those who are more cool. Yet the more you put attention onto those emotions, the stronger they become. If you do not pay attention, in the end, they will fade away. Emotions are only on the surface, thus they are an obstruction at the beginning of zazen. You can read about the naikan method of Hakuin on how to calm your emotions down. Please read those carefully.