Yeah, maybe that'll help. When people get stuck in a depression mode like that, it's very hard to shrug it off. In 12-step programs, they have a saying for people that are stuck in a funk, "This too shall pass". In other words, a person just needs to remember that eventually that particular depressed feeling will pass, and they will be onto feeling better.
But in 'Guy's' case, that feeling keeps returning with much frequency. Guy, somehow you may have to figure out a different way of looking at life? A 'new pair of glasses' so to speak. This could be more difficult than just saying it here on this thread. How to exactly do it is another story, for sure.
All the ideas mentioned are worthy of looking at, I think. Also, maybe starting a 'gratitude list'?. This is a piece of paper that you write something down on every day. Something to be grateful for in your life. [Re: A roof over your head, food to eat, a dog who loves you, etc., etc.] You write a thing or two down on this piece of paper every day, and when it is full, you move onto another piece of paper, and tape the old one up on your wall. Eventually, you begin to start wallpapering your room with grateful lists. lol... The point is to find a few things that you are grateful for - that not everything in your life is honeysuckle.