Canada

Backing off a campaign promise because you come to the conclusion that it isn't really feasible...

I'm sorry, but I do not believe that's what happened.

This article gives a timeline of the events in line with how I recall things (and why I don't accept that the failure to reform was the result of a good-faith attempt).

Also if you watch the electoral reform segment on Nathaniel Erskine-Smith's podcast interviewing Trudeau, he quite literally says that PR survived the committee process further than he had hoped, and he had to put the brakes on electoral reform against the recommendations of the committee and experts because he personally was very against PR.

"I ... had been very clear with caucus ... how much I am opposed to the idea of proportional representation ... It was something that I had to leave a little bit of a door open to, and unfortunately, because of that, it got further. ... I was not going to let that move forward."

Here's an excerpt from the article:

Part of the Liberals’ 2015 campaign promise was to create two mechanisms to ensure democratic control over the process of reforming the electoral system. The first was the all-party House of Commons Special Committee on Electoral Reform (ERRE), which exercised representative democratic control: it guided the process, investigated the alternatives in depth, heard from expert witnesses, ensured cross-party support, and enabled parliamentary oversight. The second mechanism was made up of a range of public consultations that ensured a degree of popular democratic control: an e-consultation platform, town halls across the country, mail and phone surveys, and petitions that gave all Canadians the opportunity to participate in deciding on a new electoral system.

By the end of the process, both mechanisms favored proportional representation (PR): an electoral system where each party receives a percentage of seats in parliament equal to the percentage of the popular vote they win in an election. Based on its own investigations and the testimonies of the vast majority of expert witnesses, the ERRE recommended a referendum with two options: the current system (FPTP) or a new proportional system, to be designed by the government and explained to Canadians by Elections Canada in advance of the vote.

Public input similarly favored PR: the e-consultation platform showed a strong desire for change and support for most elements of a proportional system, and other avenues of public participation similarly backing PR and a referendum. Together, these representative and popular mechanisms provided the government with a mandate to give all Canadians the final say on whether or not to switch to PR.

And sure, I suppose that he was "told repeatedly that it wasn't a good idea", if you count liberal party appointees, and discount a non-partisan committee and expert opinions:

However, when the ERRE committee released its final report, the Liberals immediately began backpedaling. Maryam Monsef, then minister of democratic reform, rejected the report. Monsef was soon replaced, but the new minister only doubled down, claiming there was no consensus for change and defending the existing FPTP system.

If you're willing to forgive this stuff, that's fine, it's your call to make.

But how this process unfolded convinced me that the electoral reform campaign promise had never been anything more than cynical manipulation of a very engaged interest group of voters, and the failure of the process was very messily engineered to provide cover for Trudeau to back out of it.