What is it about difficult situations in life that cause people to feel so depressed and to lose their chayus or their positivity? 
I think part of the reason is that all of us imagine how we would like our lives to be. We imagine ourselves growing old with our spouses, taking vacations, traveling together in our older age, enjoying our children and grandchildren and simply living life in the way we long to. We never are prepared for things not turning out the way we imagine.  Especially in America: people are used to having what they want.  In Israel people are used to constant emergencies, bombs falling, terror attacks and peoples lives are turned upside down all the time, Hashem yirachem. But most of us BH cannot relate to that.  So when things do not go as planned or as we wish, we suddenly are unable to cope. We feel sad. We feel that we are missing all those things we looked forward to. All those things we imagined happening. Because in a way we build our lives based on imagination, not reality. We concentrate on what we lack, not on what we have.  (The truth is part of not having what we want arouses in us a certain feeling of why didn’t we appreciate the good times when we had them: that is actually  not a negative phenomenon. We perhaps have to go through that as a form of teshuvah, as a way of arousing greater appreciation for the blessings G-d has bestowed upon us. But we need to get past concentrating on the past and learn to live in the present and appreciate what the present is, even if it is not what we would like. Harder said than done but necessary to work on)
We must understand that there will always be things we look forward to in life but if the plans change, that means G-d does not want us to have those things, for whatever reason. At least not at this time. And therefore rather than sitting down and feeling sad and depressed and concentrating on what we are missing, we need to learn to rise above the occasion and utilize it as an opportunity for spiritual growth. We need to learn to appreciate whatever blessings we have. And we need to see what it is that G-d wants us to appreciate and look forward to instead of what our own agenda is.  And to understand what is wanted from us at the time. Because that is far more important than yearning for what we want in our imagination. We need to learn how to want what G-d wants. This is His world. His creation. What we want is not the main thing. It is what G-d wants and what He wants for each one of us. 
Feeling a yearning for something that is not possible at the moment is a futile and frustrating experience. We can pray. We can ask for what we want. But if does not work out the way we plan, we need to let go and allow ourselves to experience what it is G-d wants us to feel, see, experience and understand. And not to feel devastated or jealous when we see others having what we wish to have. 
From the very darkness itself comes a greater light. The greatest moments of growth and closeness to G-d are precisely in those times when things do not go the way “we” imagine or plan. In precisely those moments of disappointment, depression and longing. And those emotions are perhaps what we are meant to feel because it is part of some tikun we need to go through. And that is what our neshomahs need at the moment, not traveling the world or going to a restaurant.
And the sooner we are able to appreciate that and to make ourselves happy in whatever situation we find ourselves, the sooner we will feel free from negative emotions and jealousy and we will stop feeling we are missing something. We will understand we have exactly what G-d wants us to have and what we need at the moment.  So we are not missing anything. We are fulfilling what our purpose  is in G-ds creation.  And if we are going through difficult or challenging times and we see others happy and enjoying life, we don’t have to feel that G-d is angry with us or that we are somehow less loved or privileged. We need to know that the challenges are also part of G-d’s purpose and He loves each one of us with the challenges He gives to each one.  G-d loves us, G-d knows what is needed for each one of us, and G-d is there with us in our suffering.  We must never forget that. 
And let our deepest yearning be for Moshiach and liberation from this long galus. 
That is the only truly positive yearning in this world.
May we merit the Geula now!