why does my depression go "in and out"

hey, I am 22M and had depression since I was 16.

life was going okay, with me not being depressed in the past few months, it must be linked to meeting my gf on july this year, during that time I was super easygoing, would go out everyday and shit, like life was all right the thing is that life was okay this way too last year, for a period of a month or two like this time.

But in the past couple days, it feels like everything I tried to build vanished, my easygoingness (?) left, as well as my ability to go outside just for fun or exploring, and even talking to my gf feels impossible rn, like my mind is blank with no thoughts and even if anyone started a convo with me I'd feel trapped and don't contribute at all

i have a really hardtime with how impulsive my mood feels like. I feel like I don't control it and I hate it, it makes me long for the next time I'll be a happy carefree butterfly

For a long time and still today do I think I have bipolarity disorder, it even runs through my family on both sides of my parents. But my psychiatrist is calming me down on that and keep saying that we should focus on other subjects other than bipolarity granted I may not be bipolar, but then what leads to these short moments lasting only for a few months where I feel on top of the world, feeling like everything will be fine and basically being my best self ?