As alluded to in your OP, I think social media has more to do with our moods and depression than most might think. The pictures others paint on Facebook are far from the reality of most of their inner lives. I believe if people were truly as happy, joyous, and free as they presented themselves to be - they wouldn't be posting on Facebook.
You already know my rant on chemical imbalances and medication, so I'll save the derail...
FWIW, I still go through the same as you describe, and realize it might look very different from the outside. I do my best to keep things positive here, but I have my days (sometimes weeks and months) of despair also. I learned that sadly this isn't the best place for me to air that stuff out. Thankfully I have other resources, as I know you do too.
The only thing I honestly know that works 100% of the time is helping others. I find when I'm reaching out and doing things to help other people, it's virtually impossible for me to be feeling crappy about myself. It has a cumulative effect too.
I want to say hang in there, but that sounds almost demeaning, as I'm certain you will. And I inderstand that's not what you're looking for. I will say - you're not alone. And your post achieved its mission. I needed to hear it

. Thanks.