I don’t even know what to do with myself right now because of how intense this feeling is. I (30M) am having memories of my childhood dog, who passed away in 2013. I’ll never ever forget him, and I suppose that’s bittersweet. Sometimes I get down for no reason at all, but now it’s just my sweet dog.

Kinda sounds like depression. That sensation of not being happy for some undetermined reason.
It can help to chat with someone that you trust, or if it’s getting in the way of life, even chat with a doctor if you can. But you’re not alone, I’m sure many of us can relate. It’s ok to talk about it, people who you can trust will often understand.