Finally, brother and sisters, rejoice in the Lord.
Easier said than done for most of us! We're not naturally given over to rejoicing, or so it seems. I remember being sent on a problem-solving course when I used to work in R&D. Now in research you have to have a fairly positive attitude towards problem solving. It is, after all, part of the staple diet of research. And it seems to me that in order to be truly creative about solutions, you have to be able to see the good in even the most absurd suggestion because you never know where it might lead you.
Maybe the solution to the problem of rejoicing is to be found in the ability always to see the hand of God at work. Paul certainly had enough evidence to the contrary. He was in prison, he had been beaten, his fellow Jews had tried to murder him on more than one occasion. His closest friends had abandoned him, his co-worker had been so ill they had nearly died, and those are just the things I can think of at this moment!
And yet Paul goes on about joy in this letter to the Philippians. fourteen times, as I recall, he mentions joy in some way or another. I don't think Paul was given to superficial optimism, I just think he had a perspective that allowed him to see God at work where others might not.
If you struggle to rejoice, to see even a glimmer of hope that God might be at work in your situation, then perhaps you need to learn to begin your prayers with a little thanksgiving. Give thanks to God who he is (just as Jesus taught us to pray), give thanks that you are not defined by any situation but by the grace and mercy you've received form the God who loved you enough to die on your behalf.