When I think about going to work tomorrow, I really feel depressed.
Literal
Tomorrow work [to-に] go [quotative-と] think-if, I [topic-は] truly mood [subject-が] sink.
~と考えると uses the conditional と — 'when I think about X.' 気分が滅入る ('mood sinks') is an expressive phrase for feeling depressed or gloomy, with が marking 気分 as the subject of the emotion. 本当に ('truly') intensifies the feeling.