1. Lighten up. High does not have to be loud and it sounds worse when you 'push'. Back Off.
2. Keep your throat relaxed. 'Drop' your larynx before you start singing and keep your jaw loose and unstressed.
3. Maintain strong breath support. Expand the rib cage, hold it 'open' and keep it strong. Imagine you are pushing outward with your ribs, holding that steady while you do proper diaphragm breathing. (fill the lungs/belly with air, expanding tummy out, then expel the air slowly by pressing inward/upward with your diaphragm.)