Breakups are hard in general. We all go through the stages for different durations and may experience different levels of pain during each stage. The grief experienced in a particular stage is subjective and varies for each individual.
Some people find the initial shock and denial very difficult to go through, especially if they did not initiate the breakup. Some think that the final stage of acceptance is the hardest as you have to be very patient with yourself and let go of the past. Hence, be kind to yourself and understand that we all process grief and heal from our pain in different ways.