Toxic positivity is a negative relationship with negativity
The key to true positivity is a grounded, balanced positivity. Neither rejecting negativity outright nor wallowing in it, but respecting its place in this world of duality.
For example, when I used to start feeling depressed I would down a bottle of cabernet, light up the black candles (literally), set Leonard Cohen or Nine Inch Nails on repeat, sleep until noon, and turn my general corner of the neighborhood into a shadow realm of sorrow for days...
Naturally, that didn't feel so great and it took a considerable toll on my health, so I eventually gave it up for some sunny positivity--
Instead, I would put on some upbeat flamenco music, burn a ton of white sage, and blast everything with reiki while chanting some positive affirmations. Well, that certainly seems nicer and healthier, but it was also obsessive, controlling, and based in fear: fear of falling back into that old shadow... and it also wasn't lasting without constant repetition and upkeep, which made it start to feel inauthentic and ineffective...
Now, I'm finally much more (but not perfectly) centered. My general outlook is that positivity is a long-term movement with daily fluctuations, and so I worry less about the transient presence of negativity. When I feel down, I'm more likely to go ahead and honor it by listening to a few songs by Coldplay. Maybe I'll sit down and take a 15 minute break on the couch with some tea and a deep mood. Then I'll eat something sweet and get doing something productive but easy like washing the dishes, organizing my desk, bringing in some firewood,... and I'm back in the long-term movements of positivity.
Of course if something truly terrible happens, I will be broken for a longer period of time and just have to deal with it as best I can. But as an example of daily life, it really can work something like this... even for someone like me who leans towards depression (but without being clinically diagnosed as such).
It's about finding all the small, practical ways to take back some conscious control from our natural negative-bias (which has probably been amplified by our social and lifestyle environments) so we can make more balanced choices and develop more balanced mental habits. And this is not an "always look on the bright side of life" kind of thing... It's a clear-eyed acknowledgement of the functional place of negativity in our lives combined with the rich enjoyment of pleasurable moments and harnessing the motive power of positivity to take actions that can lead to more successes in this challenging world.
For some of you this may be a simple adjustment in lifestyle habits, or it may involve therapy or medications. I even offer a course of my own HERE in which I share my recommended playlist of analytical and practical tools for rebalancing your positivity/negativity index... but whatever you end up doing, I hope you find your path to a more pleasant space for living and breathing in.