The feeling that Poland isn't strong enough to check the Soviets might be the result of a worse performance by the Poles in the Polish-Soviet war and consequentially there is more pressure on Poland to cooperate with Germany. Perhaps Locarno ITTL forces Poland to revise the border instead of just leaving the door open to negotiations? Similarly since Poland would be considerably weakened and not viewed as capable of resisting the Soviets there might be different, less favorable borders drawn in 1920 as well as some revisions to the size of the Reichswehr just in case of further Soviet aggression, since the Soviets might be more confident thanks to seizing more territory and probably taking a harder line during the peace negotiations.
Honestly it is hard to say what the reaction could have been since it could have gone a few different ways, but my gut says a worse defeat in the Polish-Soviet war that removes Polesia from the new state and a more powerful/prestigious USSR on the eastern border would humble the Polish leadership and make them a bit more willing to compromise and deal rather than thinking they could beat bigger states in a war and assuming a more belligerent posture as a result. Probably the military leadership loses quite a bit of prestige and confidence as well that would otherwise have had IOTL and avoids the 1926 coup that put the military in charge.
The next question is what is the specific POD in the war and how do the Soviets react to their victory?
I'm thinking that the POD is the Soviets are more cautious in their advance during the Battle of Warsaw, so aren't defeated like that, but are still checked and both sides end up getting exhausted and cut a deal.
Perhaps Pilsudski's resignation letter is actually accepted ITTL, though just after the war?
en.wikipedia.org
That would have all sorts of knock on effects. Also a peace of exhaustion that leaves a much smaller Poland would also effectively ensure a crippled Polish military and arguably a nation with far less confidence and more fear of another go around with the Soviets at a later date. I'd think Poland would ITTL be more susceptible to external pressure from the west regarding trade deals with Germany and negotiations at Locarno, which should remove some frictions that developed IOTL.