The Kursk incursion took so many troops from the defense of the Donbas that the front lines there can no longer be held up. It has also diverted scarce artillery ammunition which the Donbas defenders now lack. The Ukrainian defense in Donbas is currently crumbling with the Russian side taking several villages per day.
There was some hope expressed that Russia would divert troops from its Donbas attack towards Kursk. But the Kursk operation is on Russian ground where the Russian army is allowed to use conscript unit to defend the country. Conscripts are not allowed to take part in the operation within Ukraine. This gives the Russian army a large reserve that it can use against the incursion.
This war has been ongoing as a conventional conflict between two armies for 30 months with no end in sight. If the US had invaded Mexico would that conflict remain conventional for 30 months even if say China supplied money and weapons.