Yes, I'm quite positive they will give them a much happier ending compared to what we had in the book, because if they would go further into that PTSD issue, they would need an extra movie to show us all the aspects of their recovery. In books is easier to proceed faster when you want to tell something big (that happened in a very long time) in just a few lines, but in movies you can:
a) make a montage - but that's a very 80's thing
- or b) you need more time to tell everything, period. No way they can show us their growing back together AND Katniss depression in just 10 minutes (if not less). So yes, I'm sure they will pretty much skip that phase (her depression).
But that being said, I don't think the book didn't have its happy ending. To me the fact that they grow back together and in spite of everything they found each other again, means a lot. Katniss' final speech tells it all:
“What I need is the dandelion in the spring. The bright yellow that means rebirth instead of destruction. The promise that life can go on, no matter how bad our losses. That it can be good again.”
Being optimistic about a future where you believe you can be happy again, even after all the pain they suffered is impressive! Sure, they will always have their post-traumatic symptoms, but that's what life is, a compromise of what is good and what is bad and those final lines where full of hope and positive vibes to me. So even if rushed and a little bit bitter (but how could it not be?), their closure is happy in my opinion.
