The vocal folds (chords) are a muscle, and just like a muscle it takes time and practice to get the chords strong enough to sustain sound at higher pitches. If you push sound into your throat (pharynx) instead of allowing your voice to stay grounded in the belly (diaphragm), the notes will strain just like if you put too much weight on your muscles at the gym.
The solution may be to keep the sound grounded with a good breath and sustaining the sound down, as opposed to straining to hit the notes at the top. Over time, just like with any other muscle, working the vocal folds out healthily will naturally become able to sustain higher and stronger sound. Patience is the key to see permanent and lasting results.
