For those who have passed away, all we can now do is pray and supplication offered after every Azan and farz is listened to the most. In times of loneliness too, pray for their bakhshish, saviour from azab e qabar, a place in Jannat ul Firdous, all small and major sins forgiven, all that they have done publicly, privately, individually and collectively. It is hard to digest but we really do not know anyone but our own selves so it is best to not leave anything out while seeking their forgiveness. And remember, one needn't pray for just one parent; the living one's counter can also begin now.
In addition, do something fruitful in their name so they get the reward. Something fruitful that will benefit others - light in darkness, educating, water filter, etc.
Finally, pray to Allah to give you such 'khalaf' i.e. those will pray for you once you're gone, also.
Having lost both of my parents within a span of six and half months, I can well understand your condition and only pray Allah gives you patience. The secret I had was to have full faith from the core of my heart and brain that everyone has to go. 50% of the problem got solved itself.
