How to break a stalemate?

I was playing as Hungary , doing the Austria-Hungary path , I successfully took over Czechoslovakia and Austria (Czechs submits while the Austrians was invaded after the refusal). I invaded Romania after the "Claim Transylvania focus" I occupied whole of Transyvania and managed to advance into the Romanian low lands . but the supply is terrible . I tried building railways and setting everything to max motorization, but it helped very little . I can't get air superiority because I was building med tanks and switching any of my mils to use on air will not produce me enough equipment + the uk joined tthe war and I couldnt outproduce it anyways . moreover poland joined the war on their side . what do I do?