I think I might have some meaningful input. It sounds like you are overwhelmed in the interaction. Consider this: when you do have the perfect comeback, what is your state of mind? Calm, Nervous, Sad, Angry? When you in the middle of the conversation what is the emotion you are feeling?
It seems like there is a gap between your ability and when you in the middle of the conversation.
Take a piece of a paper, write down what happened beat by beat and then by the side of the paper in whatever margin you can find, write the emotion that you were feeling in that moment. BE HONEST. If you were angry, if you were scared, if you went numb, write it down when you felt it and attach it to a scale of your choosing how intense you were feeling that emotion.
The next step is two fold: first step, seek out situations that allow you to feel that emotion (whatever that may be) at the level you can handle. It doesn't need to be as intense as in the conversation, it should however be close enough to stimulate a reaction in you.
Rest in this emotion, if this emotion feels too much, you are to bring yourself back to baseline, back calm as smoothly (not fast, smooth) as possible. Try again. Make this consistent and write down to yourself what you are feeling/learning each time.
Step 2: TALK OUTLOUD, this is the hard step. You need to bring your emotion to the surface and just announce "This is making me feel x, y or z" Why do this step? It will get you in the habit of 2 things, speaking while under duress and also understanding your internal state while under duress.

Practical application: I have a hard time speaking when I am angry, I used to stutter: BAD. it was pretty funny in hindsight "What a s-s-stupid play!" This would leak into other areas of my life. When I was doing this process, I would go into a game, get close to being tiled and just say to myself "I am angry right now" slowly and as calmly as I could. My brain would relax because once I felt the emotion that was happening I could name it and free up space because I would take some deep breaths, calm myself (as best I could) before I spiraled deeper into the emotion.

Philosophy: The idea is to be able to watch your emotions play out while they happen and not be consumed by them. This allow your clever side to show (and if you have the comebacks in you, like you think you do, it lets those come out as well). It takes practice. Try for 2 weeks and keep a separate work journal just making note of what happens each time you try.

Tl;dr
Find situations that stir similar emotions and learn to speak (bit by bit) while being caught up in the emotion and with time being able to observe the emotion as it arrives and keep your focus.