I don't know if this may help you or not, but...

Let's pick that example of the manager and arriving on time. You have built two scenarios. In the first one you are "less important", in the second they are "lazy". I can see a couple of problems here.

The most immediate one is that there exist other possible scenarios. These may go from one extreme to another. I mean, I could imagine your manager being "evil", doing it on purpose to assert their authority over you, to make you feel who is the boss. I could also imagine some other extreme where your manager is giving you leeway for you to be the one who calls the meeting; they give you time to prepare or to tell them when you're ready. These scenarios may or may not apply, of course -I do not know your situation-. But they are not impossible. And in the same way there may be other possible scenarios.

The second problem derives from the fact that you focused only on those two possible scenarios. What do those two have in common? Both are negative. They put the reason for what is happening either on your manager's character flaw or on your own lack of importance. One might guess that you arrived at these scenarios by "looking for a problem". Given that you were looking for something negative, you only arrived at negative scenarios.

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What could be done?

You could avoid arriving only to negative scenarios by avoiding looking for "a problem". Looking for a problem easily ends up finding one in yourself. And then as a defence mechanism some other scenario will appear by trying to "shift the blame". In fact, your second scenario almost feels like you came up with it as a response to find a reason so that the problem is not with you but with the manager. In any case, if you start by looking for a problem you will end up finding problems.

Instead you may try two different approaches. In one you force yourself to consider that the originating reason for this situation is not -or at least may not be- a negative one. You force yourself to come up with scenarios where there is a good intention or a positive motive, even if the result is one that irritates you. You don't even have to believe these scenarios are real or correct, just allow that they might be possible at least as much as the negative ones.

The second approach goes one step beyond this. The idea is this one: So there are a number of possible scenarios, but you don't really know which one is the "correct" one. Ask yourself: Do you need to care? That is, does it really matter what is the real reason this happens? Sometimes you may need to care, sometimes not really. This depends on you, mostly. I mean, the delay on the meetings may be important to you but not to me. Or vice-versa.

One small piece of advice here: Sometimes you may think that you do care, that it is an important matter and that you want it solved/fixed, but if you give it some calm thought you will find that you actually don't care that much. So just spend some effort here identifying what is really important and what is not so much.

Either way, you may care enough to want it fixed -or at least to go further- or you may not really care that much. Now, I focused on you caring about it, but there's a second factor you should consider: Can you actually do anything about it? That is, given the various scenarios and possible reasons, can you act on any of those to change them or are they all external/out of reach to you?

Now you have four possible outcomes:

- You don't care that much, and you can't act on it. Then just accept it as it is and go on. You can't do anything about it but you have also learned that you really didn't care so much, so this is something which shouldn't bother you.

- You don't care that much, but you could fix it. Then it's mostly a question of "choosing your battles". you'd have to see how much effort would it take to fix it and balance it with the possible benefit. The benefit will be generally small, because it's not something you really care about. Either way, if it's worth it or not, the outcome should be satisfactory. In one "you don't gain much but it didn't cost you much either", in the other "you don't fix it because it cost too much, but you didn't care so much about it anyway".

- You do care and you can fix it. Then do fix it. It's all in you hands, right?

- You do care but you can't fix it. This one is the problematic one. In a more stoic approach you may choose to "let it be". Accept that you can't fix it anyway so "learn to live with it". Sometimes this is enough. Thing about the meeting situation. You won't be able to change it, all the reasons you find for it are out of your control, there's nothing you can do... but you can still choose not to let it bother you. You might choose to use those minutes for something useful, like mentally preparing yourself for the meeting, or checking the list of things you want to address so you don't forget any, etc. The situation hasn't changed but you have changed what you make of it. Of course, this doesn't always work for everyone, so another approach is this: make it so you can actually fix the situation. I mean, all the possible scenarios you've thought of are... well, in your mind. So, a first step would be to investigate the situation. You may e.g. watch your manager's behaviour with other people: is it only when meeting you that he is late or is it with every co-worker/non-customer? May be it happens with some but not all? What do those do differently? Or maybe your manager is actually expecting you to remind them of the meeting? Maybe you could try doing that once and seeing how it goes?

In any case, my advice would be a mixture of both approaches. Make an internal and honest effort to just accept that you cannot change some things and make the best of how things are. But still keep your attention on identifying things that you might actually be able to change.

The background effect this approach has is that you learn to look for many more possible scenarios other than just "either it's a flaw with myself or I can blame it on someone else". You learn to accept that sometimes it doesn't matter that much why something is the way it is, and that you can still make something out of it, and you also learn to give "positive reasons" a change as the origin of a situation.

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Im not really sure this can help you much, but I hope it does at least a little.