I posted this in a different topic, but I think you might find this helpful too:
I wouldn't consider myself an expert at all, but I have been screaming for 7 years. When you are doing vocals of any kind, it is extremely important that you do the kind of vocals that are 'comfortable' for you. Trying to sound like a certain vocalist is great if you are starting out and finding your way around your own voice, but if you try to do what isn't natural for you, you're going to hurt yourself. It sounds like you're trying to force yourself to do something your voice isn't comfortable with. An important part of vocalizing is also getting comfortable with your own voice.
Example: me. I've put out 3 releases with ORCumentary. My first album was high screams mostly, the next EP was leaning more towards lows, and my most recent album is mostly lows. Over the years i've learned that my strength is low screams and only do highs every now and then.