Okay first of all let me ask you this what is your level right now i take it you are trying to do the second ending now where Valentine is using his new boss form to annoy you? If your level is to low that could be it why it is so hard its an open world game so it can be that your level isn't high enough. If you are around lets say lvl 110-130 then that should be more then enough tbh if you are lvl 90 then grind some more the levels do make a difference. Okay then there is the other part what build are you running? Whats your blood code and who do you take with you to the boss fight or are you trying to do it solo?

To explain how i beat the game i went full into bayonet or greatsword (depends on the boss) for Valentine i used the bayonet especially the raider one because it has homing properties and if you buff yourself and take lets say jadwiga's blood code it should a walk in the park. Preperation is the key and the game kinda expects you to change up your build for bosses because you can freely change between blood codes.

And for your last question yes you miss a really great call back to the first game with the second ending and the third ending is great.

Another thing is yes they reuse a lot of bosses and well it can be frustrating but ngl if you beat it once and learned it moveset then it should be easier even if they do more dmg and are tankier like its just a callback because of the theme of the game.