I hate feeling this way

I just hate always feeling sad. I wake up sad and I’m just sad throughout the day to where I don’t even want to go hang out with my friends. It’s not that my life is bad right now, because it’s not I have a lot to look forward to right now. I’m gonna be going to college and I’m very excited for that. But I just can’t stop thinking about my past. These memories I have I want to go back to sooooo bad. I think about them constantly. Different parts of my life I’m just dying to go back to. I’m graduating high school this year so maybe this is why I’m feeling this way so bad. Certain parts of the day, smells, songs, just anything take me back. Life is just moving so fast and I can’t describe how badly I want to go back to certain times but I’m tired of feeling this way. Down all the time. I can’t seem to focus on the now. Does anyone have any advice or struggle with this too?