I messaged my ex girlfriends new fling to plant seeds of doubt about her within him.
Why would you do that? What will your ex's new boyfriend feel?
He'll feel doubtful and paranoid about her.
You seem to be having a difficult time getting over your ex but it is wrong to get in between other people's relationship. You really should try to move on from her.
I don't care what you do.
You really need to move on from her. Trying to sabotage their relationship is petty and wrong from you.