The song The Line by Twenty One Pilot actually fits Orion/Optimus and D/Megatron perfectly well. The lyrics match both of those two and how the changes they endured shattered their relationship.
Keep the memories of who I was before.
This line hit me harder than the others. Optimus and Megatron likely kept the memories of the other before everything went south.
This one is painfully accurate. They'll ask each other if their actions disappoint the other, already knowing the answer.
This last one feels like thousands of knives stab me. This line not only goes well with Megatron, who will commit terrible war crimes, but Optimus as well. In many series, Optimus is willing to sacrifice any chances to restore Cybertron to save Earth. To Megatron, Optimus has a cross line that he won't forgive.