Hi Siwee
The discussion chapter is the hardest one of all, so not surprising you're having difficulties with it. I'm also writing this chapter, and am in social sciences, so not sure if my approach is useful for you...I've put up butcher's paper all around my study, and have gone through all my chapters, written the findings in categories, then brainstormed conclusions. My individual chapters discuss my case studies and what I've found, then my discussion chapter discusses the commonalities and differences, and relates it all to the theory and to the literature.
Prepare it like any other essay - do a plan, summarise your findings, draw them together. It's hard, but keep going! Good luck!