Seth has loved horus too. Also horus isnt blaming seth. He's the only one who knows seth the best. The reason He's having a melting down is cuz of of his fear of abandonment as he knows that even tho seth had finally started to love him seth is a man of honor and loyalty and till now horus isnt 1st of his priority. On top of that he assumed seth knowing neph never cheated will surely try to take neph back but in the chapter we see Seth's reaction wasnt even thaaat extreme when he learned horus was hiding such a big information. But when mirror said horus love is fake thats when his eyes go vacant and he's become shock-daze/ crushed disappointed n wanted to run away to neph. He wanted to escape this heartbreak n find refuge in nostalgia familiarity thinking atleast neph wont be fake like horus. Also he didn't think of breaking up. He kept saying "we'll take abt this later" but poor bub horus misunderstood in his anxiety n fear.
https://www.mangago.zone/home/mangatopic/20010046/ u can check the second reply on this thread where i explained seth neph marriage circumstances
Yes everything osirishit's fault!!
Now it's all only about Horus feelings....I understand that he loves Seth. But Seth never loved Horus in the same way. He loves Nephtys since forever. It's Osiris fault everything shaddered into pieces..... and now baby Horus is blaming Seth.....