A reframe of: "I'm frustrated that X isn't including me in the room" to "What value would I add to the room that X may not realize?" works wonders.
Then, share your answer with X and see what they say. (1/5)

The more specific your pitch, the more effective.
"I will provide a design perspective" => vague, hand-wavy, what does this really mean?
"I will provide feedback on ease of use and multiple alternative paths we could consider" => hell yeah, that's valuable, get in here. (2/5)

If your pitch doesn't work, there are 3 possibilities:
1) You'd actually contribute a lot, and X would see that if you communicated it better.
2) You'd actually contribute a lot, but X can't see it due to incorrect assumptions
3) You'd actually not contribute much.
(3/5)

How to tell which one of the above it is?
Start with assuming 1. Work on how well you communicate what you'd be able to contribute. Test it on some trusted friends or colleagues.
They might be able to tell you if the real answer is 3.
(4/5)

If you get to the point where you're convinced that yes, you would actually contribute meaningfully, and yes, your pitch is convincing, then you're probably in scenario 2.
In which case, the best thing is to leave. You'll be happier in a different environment.