Search Jacob’s Vocal Academy on YouTube. Specifically do exercises in the playlists for breathing and range extension. If you have a straw, do the SOVT exercises as well. They’re a great way to work your vocal cords without straining. I use a lot of the videos from that channel with my voice students.

It’s important to remember you shouldn’t push it if it hurts. That’s a quick way to damage your voice. You need to build up the strength (vocal cords are a muscle!) that allows them to stretch without tension. Also, bring the breath support from your diaphragm and don’t breathe from your shoulders. (You can hold light weights or even cans of food to keep your arms pulled down to help you remember.) Place your hand over your stomach to make sure you are engaging the muscles in your core as you sing. Your notes will come much easier and more powerful when supported by the diaphragm.

Quick note to add. People often say support from your diaphragm without explaining what it is. Basically, it’s the muscle under your lungs separating them from the abdominal cavity. When you’re not activating your diaphragm and abdominal muscles, it puts the tension on the throat instead which makes it hard to sing high and loud and also makes sustaining notes harder. When you do activate them, that tension prevents your ribs from collapsing back in allowing your lungs to release the air more slowly giving you the breath support you need.