I was in a similar boat, but recently started getting through that barrier. The thing that clicked for me is pretty simple: I was filtering myself and chipping away at that filter made a huge difference.

For example, I was in the elevator with a neighbour and they were carrying a lot of mugs. I said "that's a lot of mugs" and we ended up having a quick conversation.

In my case at least the conversation starters are all there in my head, but I'm discarding them hunting for the "perfect" one which obviously never comes and the moment passes.