Understanding life

I've been trying my best to keep positive, do the work, change how I think, do more, be active, be optimistic. But how can I shake off the sadness that comes after everything I do? It's exhausting. I keep trying my best but at the end of the day iam just sad. When will life turn around and be positive, just this once, when will it be ok?

I look at life thinking if it doesn't become better, then seriously why bother?

I'm really not understanding life at the moment.