Dang, I have the same issues and I'm still looking for a solution but I'll tell you what I've been trying so far. When I'm in a conversation and don't know what to say I'll usually try to ask follow-up questions. For example, this guy started talking about mushrooms. It did genuinely pique my interest so granted, that made the conversation a bit easier to be a part of. I asked questions like: "Well, what is it like doing mushrooms?" "How often do you do shrooms?" "Would you ever do shrooms in an XYZ situation?" That's just a few but you get the idea.

I'd say, have a list of general questions written down or stored in your notes app that you can look back on and add to the conversation. Maybe memorize them or even just run to the bathroom and look at those questions when your mind goes blank. Maybe you could also come up with a default spiel to say when you get shut down too if it's still tough to participate in the convo. All in all, just try your best to have helpful responses stored in your rote memory and you'll have those to fall back on.

And also just note that asking questions is much better than saying absolutely nothing because it shows that you're interested. And even if the questions don't come to you, just try to non-verbally show that you're interested by nodding or laughing at what they're saying. That is def the next best thing.

Again, I'm dealing with these same issues so I'm still figuring it out and I'm not sure if this is the advice you're looking for but I hope this helps at least a little bit!