I am a little like your Thunder. I have worked hard at not being, but the fact of the matter is that for me the issue is two fold... first of all I grew up in a family where we were encouraged to speak our mind and communicate, but often were bashed down like a stake in the ground after if my parents disagreed. the information was then used against me and I was made to feel guilty and that they, specifically my mother, were martyrs for me. My mum still uses words to punish and inflict her will on others by telling people not to do something that she does... very hypocritical. All of it compounded on me as a child and being the black sheep and unsafe and standoutish in the crowd... I hid by not talking, becoming sullen and faking I was okay.
On top of that I really needed space and my family was and still is on top of me all the time. I am an extrovert and gather energy from the ones I love when we are all together, but even an extrovert needs down time and a door to close so as to think and catch up with oneself.
I would suggest to you that your Thunder is overwhelmed with his feelings, and the situation he is in to communicate is threatening in someway and that he feels cornered... he wouldn't stay if the dynamic your tribe is in isn't for him... he might not stay. Or he just needs some space and for people to stop bugging him to communicate when he is feeling threatened to do so.
When I or anyone I am with gets like that I don't say, "I'm fine" but "I will catch you up when I can and I feel comfortable enough to.
"I don't really know what is going on for me, but I need some space and I will figure it out... when I do I will let you know," "please be patient with me, I am working out what to say so as to not represent myself in a way that is not accurate." "I am having feelings, but don't know what they are yet and can't explain them yet. Please give me some space and time to figure it out and if it is to do with you I will let you know."
the above are things I have said... and request that others say if I am confused about their reaction to me. Then I try and let it go and wait, either to figure out what the hell is going on and whether or not this something I can talk about and trust the listener, or if it's not that important, or whether or not I just need some space to gather my thoughts...
I find that it really works well to tell people what I need to hear from them. I don't like to assume that people know the words I need to hear... I quite often ask PN to tell me something in a certain way so that I experiencing hearing it in the way that makes me feel comfortable and patient.
You could do this too. You could request that he tell you he needs space, or that he is figuring something out or what ever it is that is going on for him. You don't have to know details right away, but clearly he is not okay and you want to know why, because you care... when he doesn't offer you a reason it is like he is disrespecting your support, caring, love for him. At least this is what I experience. It build resentment and eventually becomes something it doesn't when all that might be said from him is that he is going through something but isn't in a position to talk about it yet.
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