Mahinda was determined to win this war because of one main reason : REVENGE!
The LTTE group killed his father, and thats why he was determined to end this. But other than that, there were numerous rumours about him bribing the LTTE group itself to win the Presidential Elections.
I dont think he did this mainly because he loved you or me, or this country.
This war was nothing but a business. At every bullet shot, there was someone earning millions, thats why it was never prompted to an end.
But Mahinda's intention was not to earn, because he had personal gains on this, one very strong proof of this is that Gotabhaya being the defence secretariat, Mahinda's own kin. Maybe they wanted to kill Prabhakran themselves to enjoy revenge.