Diablo 1 HD Hellfire Mod | Awake

Look, I don't know who came up with the idea to make the bosses' health bars come back completely full when you teleport to the city but this is making the game impossible to beat.
I can get to the diablo in nightmare level, but when I go to face him and I need to go back to the city to replenish my mana, when I return to hell to continue the fight his life bar comes back completely. I can't face him like that because I'll always have to go back to town to buy mana.
Please fix this. When you return his health bar should be in the same place as when you left. This thing about making their bar go back to the maximum is unreal...

There's nothing to fix. Get good.

You must be new to Diablo or at least to this mode.
It is definitely not impossible. In fact, you are not even half way through the game...
If you need the entire belt AND the entire inventory full of mana and still can't beat him, you are just not strong enough yet or your strategy os weak. There are several posts in this forum on how to clear all levels and difficulties of the mode. Moreover, there are videos on Youtube showing how to beat the last difficulty with ALL characters.
You can do it, don't let the Lord of Evil prevail!!

You can drop 1000 mana on the ground, and pick it up at any time you want.

Just a question, can it be that your 1 hour time has expired?
You know, if you face Diablo after the time expires, then he's almost unbeatable even on normal difficulty.