Not sure if you ever figured this out, but I believe this is due to your equipment degrading over time. Resting at a shrine will restore all of your health.

It's the wounding effect. As you get hit your maximum health decreases. Bandaged Ring and several special healing items can help to mitigate this. Its' there to discourage you from just face-tanking everything, as it lowers your health to dangerous levels.

elemental imbalance can decrease your heathy when you cast spells, need ring "Link of Fire and Sky Effect"

you need to put some honey on your controller and shove it in your arse while sucking on a grasshoppers *****, will work like a charm

its wounding some damage can add wounding and the other is spell fatigue