"It is surely time to accept that we have made a mistake"
No, clearly no mistake, why even suggest it ?
"Whichever way we voted, things are not turning out the way we expected".
No, but there again the situation is dynamic, no one is controlled to do the right thing, and no one wanted to believe the EU would be so intransigent as to cut off their own nose to spite their face; but one deals with life's disappointments, not collapse our aims at the slightest hurdle.
"Or are we too proud?"
Pride has B all to do with it, and your attempt to use emotions to sway is noted.
"We changed our minds, thought again, brought back a king"
Well not everyone gets things right, our ancestors clearly made some howlers in the past, as you indicated; but there again maybe they felt they had little ability to control parliament at that stage and for some mad reason thought controlling monarchy would be easier.
"And we can remove governments we do not like"
This is important, we can get rid of leaders, at least in the House of Commons. Unlike the EU Commission. But one needs to keep in mind that we have limited choices as we have allowed parties to form and they don't go away, it going to be one inadequate lot or another put into power each election. We change our minds and put another elite group in to boss us around, so the system isn't truly democratic yet.
"Forming a trading club of friendly democratic nations was the only way to create a lasting peace"
Well one can dispute that thought, I certainly would. But even if it is accepted (and one then needs to ask why they want a hard border in Ireland if that interested in peace) then perhaps they should have kept it a trading club. The issue is that they did not.
"And in 1975, just to be sure the people supported the idea of belonging to this new Europe, we held a referendum"
I find that description disingenuous. If they wanted to know that, the referendum would have been held before entry, plus the lies about it being a trading club only would have been admitted to and the true nature of the beast explained. In fact all the referendum told us was that now someone has dragged us in unasked, and we are told it's simply a trading group, do we wish to reverse it already before we have had much experience of it. Folk made a decision based on trade with neighbours not on becoming a minor area in a Europe wide "nation".
"For forty years we have been members of this club"
Getting more and more disillusioned with every change signed up to by inadequate PMs.
"Pressure for ever closer union"
Gradually it became clear that this meant being one nation under an unelected elite, not simply getting common standards for trade.
"Erstwhile enemies"
Times change, an enemy one moment is a friend the next. Emotional appeals tying together peace to the EU is unreasonable. Does anyone think we'd be at war with Europe now if the EU didn't exist ? That the EU exists interfering in nations’ affairs is causing more strife than peace at present. He's just storytelling at this point.
"This country benefitted greatly from being in Europe"
Well one could not be both in and out to compare; but even, for the sake of argument, we believed that we were economically better off, it came with far too high a cost. And that is the issue, not the flowery, ”we may have made a bit more wealth; perhaps”.
"We had a shared sovereignty now"
One doesn't share sovereignty one has it or not. For sure, one can give up bits gradually, and that is what has occurred and continues to do so. But nothing is shared. We have what we still retain, the EU has what we gave away. This is the cost that is unacceptable. It is why we can not "think again" as it is the goal we were courageous enough to vote for in the last referendum, and which those who don't value democracy is trying to overturn by putting fear into any waverers.