How to deal with these feelings?

Most days I’m fine, and in a good mood and quite content with life. But there are days in which I can’t help but think about different things which make me feel pretty sad and even useless. For instance, it can sound a bit cliché, but thinking about kids with no families, or elderly people with no families, people who live in unhappy marriages/families, people with disabilities or medical conditions, people who starve, people who are depressed, even about animals that have no homes or that are mistreated… as you can see, there are many things which put me in that mood, I probably forgot to mention many of them, but you get what I’m talking about.

It’s a disheartening feeling. I am very, VERY grateful for my life even if sometimes I get down about silly things. But knowing that other people suffer so much just makes me feel terribly sad. I know I can go volunteering, and I probably will, but that won’t make much of a difference.

How can I deal with those episodes? Just about anything triggers it.