Do I have to go? How far do you go to please other people?

I think you need to invest in friendships as much as anything else and sometimes this involves making the effort if you are tired or stressed. That said family comes first and you have a clash so it's easy to say you can't go this time. I do think however when telling them this you should organise something else, meet for coffee or a walk or even just a proper chat by phone. If you genuinely don't want to do even this, then that tells you you are not invested enough in them and it's time to let them go as friends. This is fair enough but make sure its what you really want, it's a two way street and they will probably be hurt by this and may not want anything to do with you if you are suddenly feeling more sociable next year.

This. Any real friend would expect you to put your child's event first. If doing so ends the friendship, that tells you everything you need to know about the quality of the friendship.

Stop backpedaling!! You have somewhere else you need to be, just tell them things have changed and you can no longer make it. It's fine not to go.

And that too, if that's what she wants

Nothing sad about that at all, I think your New Year resolution is fab.
Do what makes you happy.

I think that's fair enough. If you're prepared to pay for your place so that no one is out of pocket, it doesn't seem unreasonable to withdraw, and you've said you'd approach such an invitation differently in the future.

Has none of you ever missed one of these events in 40 years? Surely somebody has? I think you saying the friendships would all be over if you missed one birthday sounds like insecurity, as does the fact you come away feeling bad. Are you insecure around them? Do they make you feel lesser?
It doesn't sound healthy tbh. You ought to want to see your friends, and also be able to turn down invitations occasionally without being dumped by them. These don't sound like good friends.

I have a bff who I love to bits, we've been friends for 35 years. She would be completely ok if I had to bail because DC had qualified for a sporting event (though we'd definitely reschedule the get-together).

Sounds like this friendship has run it's course. Go to your son's event and learn how to say no in future!

You can spend time with family every other weekend.

I think the time to say no to this one was back when it was booked, which was unfortunately before your resolution. In future you should decline such invitations at the start.
For this one, it’s a bit of a gray area. If you don’t think you’ll miss these friends, then don’t go. But for those saying friends who’d drop the friendship over this aren’t worth having, I’d say I have little time for friends who commit to something and then bail, so this would damage a friendship for me (though I wouldn’t drop you at the first instance)

If you're going to start saying no to things, which is fair enough, it's ALL in the timing. Say no when you're first asked, everyone knows where they stand and you're not letting anyone down. Say no when you've accepted because you don't fancy it anymore and you're being unreliable and rude and will damage your friendships. It would be wrong to pull out of your friends birthday. There's been threads of late from people's whose friends did this and they were very hurt by it.

I think if I didn't have other plans that weekend I would try to make the effort to go. These things often turn out to be more fun once you're there and I get the feeling from your post that you will only sit stewing over what people may be saying about you if you don't go.

I'm a creature of habit and love my home comforts. I really have to force myself to do things like this sometimes but I'm always glad that I did. Because mostly I end up having a great time and also because the friendships mean a lot to me and it does take a little effort to maintain them.

However......that being said, you have a clash of plans. It's your family and you'd much prefer to do this thing with them. So you have a genuine excuse. If you'd only be going to people please and you'd much rather attend the sporting event with your family I guess you have your answer.

If you don't want to go.... don't go. Just tell them something has come up and you are sorry.

Evenutally people stop inviting you to stuff - whioch suits me fine. I am a loner, happy with my own little family group.

This…

You've got a genuine clash with a more important event this time and I think anyone would understand prioritising your son. I wouldn't expect this to be the end of the friendship. Obv you can't leave the others out of pocket after confirming but as you say, that's not the issue. If they blank you then they weren't really friends, even distant ones. Send a card and gift or flowers and I think it'll be fine. I'd dodge this event but try and keep the door open from your end as 40 years is a long time.

If the friendship is over because you can’t attend then they aren’t friends worth keeping anyway.

I would put the sporting event above an old friendship meet up where nobody is really friends anyway.

I think if the friendship would be over if you don't attend a birthday it should tell you all you need to know about the friendship

But sometimes we have to do things we don't want to, because it's the right thing to do, don't we?

That applies to things like cleaning the toilet and paying taxes. Social engagements are always optional unless you're part of the Royal family.

If you lose friendships over not attending one single event, they weren't actual friends to begin with.

How do you know when you should say no to things you don't want to do, and when do you just push through and do them because you know you'd been unreasonable not to?

I think your question is flawed. "Pushing through when you know you'd be unreasonable not to" is not the opposite of knowing when you don't want to do something.

If you are committed to saying no then you need to decide on what is important to you, your values so you can evaluate each decision based on an unchanging set of rules.

What is more important to you, how your self esteem and self respect, or what certain people think of you?

I know that if I don't go, the friendship will probably be over.

So. Does that matter to you?
If so then go.
If not, then don’t go.

Your ds has got into a sporting event. Your family come first.

Just say it's come up and you can't go anymore. If they need payment and you'd committed and the price is based on that then yes pay - but don't go if you don't want to.

Well, work on the people-pleasing. It's not an attractive trait, and it means you do things like waste your time going on a weekend you know you won't enjoy with people you don't care if you ever see again, purely because you don't want anyone to have the opportunity to say anything negative about you. Is that really how you want to live?

At least you know why you're compelled to do things you have no desire to do, and aren't like many people-pleasers, deluding yourself that you're simply 'too nice' -- which is the first step to tackling it like the bad habit it is.

But no, you should go to your son's thing with a free conscience.