I feel like I'm shutting down

I don't know what's happening to me. I've always had pretty severe depression, but i finally found a good formula of mess and I've been doing okay for a few years. But recently I've been feeling really mentally sluggish, if that makes sense. It's like there's so many things I'm beating myself up over that I can't take it anymore, and my head is constantly spinning but I mostly just feel dead inside. I don't know what is going on and I'm scared, this has been going on for months and it's only getting worse. Is this some sort of a mental breakdown?

Though you say you have a history of depression, it is hard to know (and I have no medical training) if what you are describing is physical or mental (and our physical health effects our mental health and vice versa). Have you had a physical lately? I would start there. Of course, you might need a psychiatrist too. Have you gone to a psych doc before? Basically, I think bearguardian is right--ask for help so you can enjoy your life more and worry less.

It could be, are you still functional? Are your symptoms similar to these of your past depressive episode?

Find some time to relax. Do the things that you like to do.
I've been feeling the same these past weeks and I've gotten out of it by focusing on positive thoughts.
Goodluck

Have you had any new excessively stressful events come into your life as of late? Perhaps taking up a de-stressing hobby or a holiday may help clear your mind a little; you don't even have to go abroad if it's too costly.

This is what depression is --it can kill you without literally killing you (though it can do that, too), especially if left untreated.

"Dead inside."

Yep.

Professional treatment if able would be very strongly recommended.

I am functional, but it's getting harder and harder to force myself to get up in the morning. Physically I'm mostly fine though

There are things in life that depress us down, most of them can be unfelt or unknown.
Be more and more in touch with your feelings. Depression wants relief, it wants balance first and foremost.
I started with an aquarium addition to my room. Renewing my room has also helped.
Also, adopting exercising and occasional bike riding. Giving attention not just to your mind, but also to your impulsive organs - gut and heart for instance.
This is my personal experience.

Therapy in no question can greatly help.