The two words mean approximately the same thing but I find that there is a nuance I'm missing in terms of the appropriate context to use one or the other. How should I think about the difference between these two?
确定 means sure that / sure to do something.
(我确定他在中午之前一定能把那碗饭吃完。）--> I am sure that he will be able to finish eating that plate of rice.
决定(N) means decision.
（二十分钟后，他终于做出了决定。）--> 20 minutes later, he finally made his decision.
决定(V) means decide to do something.
(我决定吃饭。）--> I decided to eat rice.
Hope this helped. (: