I'm not sure there is any easy answers here. It is a big step to ask the question though. The fact the you are looking for the strength says a lot. For me, I think it starts small. Awareness often happens first. I will try to be aware of things that have gone positively. Try to be grateful for them, we all know things can go wrong and it is much easier to find the negative. It's like any muscle you want to strengthen, practice practice practice. It won't be easy, but it does get easier with time. Hope that helps.
I have learned that in the most difficult situations it is when I should most be grateful for the thing I have that makes me happy and think about the difficult times I have already passed… I can do anything! Nothing is forever!
I like to remember the saying "It's a bad day, not a bad life" and realize that even when I'm struggling, there are others out there without access to basic resources. We have the power to change our own circumstances, and for that we must be grateful