Trump wanted a peace deal and thought Zelensky would accept that the Russians won't leave Ukrainian land. Trump doesn't think Ukraine can drive the Russians out without an influx of troops. NATO isn't going to commit any ground troops and escalate the tensions between them and Russia any higher. Trump sees a stalemate, so he's trying to undo it to get Zelenskyy unwillingly to the peace table.
I don't like it - never have liked "peace at ANY cost" ideas. But that seems to be what's happening.