Both are though. I personally believe that letting go/moving on is the hardest part because it’s difficult for most people to let go of their past or a person that they have given 100% of themselves to. However, starting over isnt necessarily a bad thing. It can be exciting because it symbolizes shredding the past and showing the world the new you and usually that means that you have let go of the past and are now ready to move on.
But I believe we need these experiences in life because you can only grow from them. Situations like heartaches test you in so many levels emotionally, mentally, and sometimes even spiritually. As much as it hurts, we need the pain in order to grow.