In heaven, how can you be happy all the time if you never expeirence sadness?

so i keep thinking about heaven ever since my grandmom ped away last week she was absolutly amazing i miss her so much. she was so cute and had 7 kids , 15 grandkids and 1 great grand child. i keep thinking about heaven and how shes looking down on me and it creeps me out to think about what if there isnt a heaven? i pray to her every night but i keep wondering how can you just be happy up there forever? wouldnt that get boring? death is really starting to scare me any of the options of things that could happen just freak me out. especially if when you die nothing happens and it just ends.