Damn... Have you tried lowering the difficulty to the lowest setting?
I've been considering adding more difficulty options, including a "story only" kinda thing where you can't actually game over. You're not the first person to have trouble against the Angel, sorry about that
Hmm... The way it works is that the lowest your HP is, the quicker it regenerates. This effect is even stronger the lower the difficulty is. If you're still up for trying, I would recommend fully focusing on evasion, without attacking, whenever your heart becomes white (1 HP left) or even red (2 HP left). This will make it easier to regenerate, and you'll be able to observe the patterns as well (In fact, you could even spend a try or two just observing and learning, without the extra mental load of aiming and shooting)! Then, switch back to attacking once your heart has regenerated enough / during an easy pattern.
This is the best I've got currently, short of sharing a save file past the Angel fight and a video to catch up on story...!