I have exactly the same difficulty. I can't think quickly on-the-spot. I struggle with this constantly, so I'm probably not the best person to hand out advice. Except that I've always heard the best thing you can do is try very hard to pause before responding to anything or anyone. Take a breath, count a beat or two, in order to try to get in touch with your true reaction, feelings, answer, or whatever the situation is.
If it's something in which you have to make a decision and you don't have to impart that decision right away, ask for some time to think things over, or say you'll get back to the person.
But if it's a spur of the moment thing that needs a response right now, try the mentally stepping back and taking a calm breath method.
For me, I think part of the problem is that I'm trying so hard to react instantly the way I feel I'm expected to do (because "other" people appear to do so), that I don't take a second to truly get in touch with what I really want to say or respond with.
I can't tell you I'm always successful in pausing for those few beats in order to marshall my thoughts, but it's the advice I always run into about this.
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