Now I know this sounds insane however, I will present what I believe has helped me become better at speech over the course of just a few weeks.

Throughout the years I have noticed that some of my friends who were good at "come backs" and quick witted responses on the spot and were able to think of interesting on unique thoughts from nowhere all had shared something similar. That was that they all could at some level rap a few lines and make a quick story. Now I am not saying that rapping was the whole reason behind this but I believe at some level it had helped them. Just think of how hard it is sometimes to formulate or conjugate thoughts on the spot.

If in theory you practice forcefully to train your brain at pulling together words and making sense of them in quick succession, would that not help you become better at piecing together sentences in day to day lives?

The way that I train is by how it is shown in this video: https://youtu.be/RbimBJJiGUA?t=59s

A friend and I have been doing this for about 15-20 minutes daily trying to improve ourselves. From what I have noticed is that when I speak to others now it's much easier to come up with responses to unique situations. I no longer blank out as much as I used to and my replies are starting to become more interesting to listen to as well as I am able to carry on conversations longer than I have been in the past.

This is of course is no replacement to actually going out and practicing socialising with real people but simply something you could try in the comfort of your own home.

Good luck!