I am engaged but I am not sure if I want to get married to him. I heard that Istikhara cannot be preformed after a decision had been made: Is this true? Or can I still perform it and get guidance in making my decision?
You having doubts after engagement is a form Istikhara itself. If you think now that it is not the best option for you, then take appropriate actions to remedy it.
Istikhara doesn't provide any divine guidance. Istikhara is nothing more reflecting on your options and deciding for one from several options. However, we have colored Istikhara with a brush of divinity.
Istikhara is not special to Muslim. People all over do it. They call it "sleeping with an idea or a decision". If after waking up next morning, the idea is still as strong, they go with it. Thinking overnight about something in a relaxed manner, uncovers several factor which one may miss in the rush of the moment. Istikhara is exactly this and nothing more. And since we are Muslims, we couple this overnight pondering/thinking with a Dua as well.
You can make the prayer for that.
You can also make the prayer for existing things, for example asking God if this engagement is right for me help me with it else take it away from me.