Bringing up old problems

I've actually written a lot of things in the past month, and will be posting soon :)

When I hear something I don't like, I don't always say something right away because I haven't decided yet how I feel. And so if I start talking about it I’ll more than likely get too emotional and won't get out what I need to say. So then I end up revisiting it multiple times over, which I think is more stressful and annoying then telling you about it later when I know exactly what the problem is. So how do I resolve this issue? How do I make my brain go faster and understand my reactions and emotions in two minutes when I'm used to taking days to get it together?