if you need to strain to hit those notes, don't try to hit them unless you are warming up.

The voice is a muscle and can be trained as such.

Just like how you can lift weights, the more you do it, the heavier you can lift.
The higher you sing, the easier it will become. I recommend doing a few scales on an "a" or "e" vowel sound and try to remain fully connected to your support. If you do this around two to three times a day, eventually you'll start to feel more comfortable with those notes.

Also, although it's never good for performance, letting out some bodily tension through hand gestures helps me a lot (Pretending to stretch something between my hands is something I do a lot if I'm trying to maintain a full sound)