I do the same thing. This is just my take, but there's no way to avoid changing emotions. Even in the darkest depression your mood will lift a bit even if for a second or two, and likewise even the person with the happiest easiest life is sometimes depressed. Change is the only thing you can count on.

Just keep trying to pull yourself into the present moment. Take a walk and try to see how many things you can observe just to pull your mind out of the loop. I've been in a deep dark depression before, it is a thought pattern that feels unstoppable, but it can stop. Of course I used medication and therapy also, and I am still sad quite a lot, but I can see some things about it now and I feel much better now. Remember, this too shall pass.