I changed my tactics hours before a league match and a cup match. I changed from 4-2-3-1 to 4-5-1V.

I lost to a far weaker side in my league and I lost 2-1 at home and away against the cup opponent.
Imagine my frustration as before this point I was unbeaten and got too cocky.

Your opponent had 3 strikers and was a threat down the wings, so he obviously went for the jugular. His attack is overwhelming.
All he had to defend against was your right winger and lone striker.

I'll bet that you went attack, whole pitch, short passing?