I have always thought that mindfulness was supposed to be about accepting what's going on inside of ourselves, not trying to control our thoughts. "Being in control" sounds like an ego need, and if we're striving for perfect bliss, then we reject any part of ourselves that doesn't live up to that ideal.
That is what I think people mean when they talk about toxic positivity. Denying that it's okay to feel anything other than blissful can feel like shaming and judgment. For many people, especially those of us suffering from mental illnesses, it's not as simple as just letting go of negativity. If that's been your experience, that's great, but that's not how it is for a lot of people. There are deeper layers of work that may need to be done.