You have associated it with boredom, and you made me instantly realized that "ennui" is surely related to the Italian (my mother tongue) word "annoiato". Quick google search, and in fact it comes from the French equivalent of annoiato.
This means that when I translate "bored" with "annoiato", I am actually doing a (super common) mistake.
It's super interesting how words between languages have slightly different meanings, but sometimes these meanings are altered without us realizing it.
Of course forcing yourself to do that can be a very big challenge. Just don't wait for when you "want" to do it, cause that won't happen.
This ennui is not all the time, but occasional, and usually only for a period of a few hours.
Depression is a different feeling to me. Depression is more feeling incapable of doing anything, where ennui feels more like I could do something, I desperately want to be doing something, but nothing sounds enjoyable. I don't feel bored when my depression is bad, because I'm incapable of even feeling boredom.
Edit: I was also very recently diagnosed with ADHD, and am not being treated yet (dr appt in a few days!). It's possible maybe that my ennui is actually a combination of mild depression and the ADHD inability to focus? It will be interesting to see if I feel it less frequently once my ADHD is being properly treated.
The only reason they'd sneer at you is your "just" part. There's no "just" with a mental state that can leave people unable to even will themselves out of bed for days at a time (at my worst I could at least do that, but couldn't get out of the apartment without using up most of my mental and emotional strength). There are points where medication (mind-altering drugs) can be essential for people, and for others it may be something they need forever.
Why? Because there are different causes. Mine is a consequence of anxiety, addressing that essentially eliminated my depression. For others, their anxiety is worse than mine and rather than medication for depression they need medication for anxiety, and not just coping techniques. For other people, the depression is a condition on its own, with no or no apparent causal factors. Others, it may be a consequence of physical health (lack of sunlight, exercise, poor diet) and so those precise things can eliminate or significantly reduce their experience of depression.
It's all going to be case-by-case, and so there is no "just".