I think what happened with Kursk might have been some kind of a backroom deal.
I certainly don't think it was lost because the US suddenly stopped supplying intel.
The problems in Kursk started mounting already many months ago, and became critical long before Trump pulled the intel support.
But still, it's suspicious how rapidly it has all collapsed. Basically within a week Ukraine had lost territory that it had managed to defend since last August. Territory which Zelensky has many times called "crucial" for securing a "just and lasting peace". They had allocated their most capable units to the Kursk operation - another sign of its great importance from the POV of the Ukrainians.
So here's how the sequence of events looks from my side:
1) January 20th - Trump inauguration - Putin speaks at same time, and in this speech rules out any kind of potential ceasefires.
2) Trump team opens talks with Russia. Russia signals it is open to negotiations.
3) Zelensky and Trump have their famous problems. Ukraine is losing the PR game of "who's the biggest obstacle to peace?"
4) Ukraine does an about face, and begins making daily statements about how its ready for peace.
5)
Here's where I think something happened. In their secret talks, the Americans tell the Russians - "the Ukrainians are ready to do a deal. The ball is in your court." The Russians say, "We are actually not ready." The Americans ask what needs to happen, and the Russians admit that "continued Ukrainian control over Kursk makes it impossible for us to negotiate - it's a matter of face."
6) The Americans then go back to the Ukrainians and say - "The Russians won't talk unless they are back in control of Kursk." The Ukrainians reply - "Fat chance!". The Americans then ask the Ukrainians, what would need to happen for them to leave Kursk. I think the Ukrainians said that at a minimum, they would like to see a full resumption of weapons and financial aid from the United States.
7) Aid and intelligence flows are restored, and Ukraine does its part by withdrawing from Kursk. After all, for them this is a defensive war, and them controlling Kursk does little in the eyes of the global public opinion.
8) Russia receives a valuable and important signal of credibility from the United States. The US promised to remove the Ukrainians from Kursk, and it fulfilled this promise. Now, Russia must reciprocate by entering into these talks that Trump wants so desperately - even though we're not in love with the idea. We have to do this, because now we have a stake in the relationship with the United States, and it is in Putin's interest to keep Trump happy. This was not the case with Biden.
Anyway, this is just my theory on the rapid collapse of Kursk.