yes, that saying does carry with it the implied "provided the moment is good". Seize the good times when they come without allowing stress over the future to cloud your enjoyment of the now. That's the gist. The whole works falls apart when you bring depression into the mix. Where then the now is horrible, the future is horrible, the past was horrible. So... yeah. The phrase is clearly intended for healthy people.
For you, and for me, and for all us here at this forum, something along the lines of just... hold on, things might get better in the future, would be more appropriate.