Advice Wanted - Game lost--What should I have done?

Ok so my last game I played Morgana support . It was: Morg (me) and Vayne vs. Thresh and Lucian Nasus vs. Renekton Twisted Fate vs. Cass Panth vs. Evelynn

by mid game, we lost bot, nasus lost top, TF was doing slightly better than Cass (he was very skilled though), and Panth was making plays but dying. We were losing fights left and right. I didn't know what to do.. i wanted to follow Vayne and help her (ward, get experience from minions, keep her safe) but there was stuff happening all over the map. Everyone I ran into was blowing me up. I focused buying support items: mikael, frost queen, aegis.

I ended up just trying to follow TF, he was out most fed. I died often though, as Cass just blew me up so hard. Ideas?