Few thoughts come to mind:

You say that you tend to progress things on your own but then again you quite often discuss and process things here in PC. It looks like a bit like a contradiction to me and I obviously don't know what it means but that's just something that caught my eye. There's of course also a context why it caught my eye. I am also someone who has always processed things alone, which makes processing them in therapy with someone else very difficult. But I have very rarely felt the need to come and process my stuff here in PC because I suppose I don't believe the people here would have anything useful to say to me anyway. I'm not saying to judge either way as good or bad, but just to point out that it doesn't look to me that you always process things alone.

My experience and understanding about long-term therapy is that the important stuff will reveal themselves precisely through repetition. I'm pretty certain that everyone in the long-term therapy has some topic that comes up repeatedly again and again. I certainly do. This topic doesn't look like "problem" per se but the very fact that it comes up so often (almost every session) means that it is in some way related to the very (and probably most) important problems in my emotional life. For you this topic seems to be the email-business. I would suggest not to discount it as some side topic but treat this topic and your own feelings surrounding it with respect. It's possible that discussing this topic until it doesn't feel relevant anymore is something that allows you to make a lot of progress in therapy.

Regarding these discussions about email, do you carry on with these conversations when he answers you or do you stop there? If he told you that he will not always be there, did you tell him that you know that but why couldn't he answer sometimes when he is there? What did he answer to that?

As to why the T's says a different reason every time, my guess is that he is responding to you based on his feeling and not based on some ready-made logical-rational response. I'm making an assumption here (based on how my own T operates) but let's make that assumption right now. When he is responding based on feeling then it means that he is emotionally there with you and following you and the response stems basically from two things: obviously from the fact that he stated such a boundary but secondly based on his reactions to the emotions in the room right now. As the emotions can be different every time then the explanation can be also a bit different. It doesn't mean that the thing he said last time wouldn't be right but that particular aspect might not be salient at this moment.

When he is able to follow your feelings well enough then his responses should start to make sense, maybe just for a moment at first and I remember you telling that sometimes his response has made sense in the moment. It's natural to forget it and come back to it repeatedly. Making sense of it so-called globally, so that it would stick, takes time and lots of repetitions. A more interesting question though is: what is this problem in your life or personality that the email-saga addresses? Because I'm pretty certain that there's something very important behind it.