You can't delay the inevitable any further. It's time to accept it is about to happen. From your description, you have admitted the relationship is about to end. This is the first step in acting positively during the breakup.
Breathe in and out several times as a way of calming yourself. Tell yourself and believe so that you won't act negatively. You will cry but you won't raise your voice.
When you see him, reinforce the thought "You have to stand strong. You have to accept it. You have to respond positively. It's for your own good."
It's better you cry letting tears have their way. This will lessen the negative emotions that might explode.
You might want to look down. When you look at his face, you will let down your guard to exercise self-control. You can look down nodding your head in agreement. You can say, "Yes, I understand." And, don't talk about anything else. A day after he leaves you can text him. You can let him know you didn't talk much because you didn't want to act negatively. You will let him know you have accepted it. Thank him for everything.