How to develop my mixed voice (Because I don't have one)?

Start in your falsetto. Sing strongly down a scale until your voice naturally breaks. The pitch you break to will be sung in head tone. Then memorize what that feels like and pull in down into a few more pitches. Do not let it drop into your throat. After you have done this a few days you can begin taking it up the other direction. It should free in your throat. Both chest and head feel free in the throat when done correctly. It is where you feel it resonate that makes the difference.

To mix these start at the top and maintain a focus down the front of the face. It is called the focus or mask. Do not let it fall back. Glide down like a trombone glide. Do not let it have any breaks or bumps. Glide up and down until you get used to he fell. Then start plugging in pitches.

Good luck.

A good voice coach will teach you "mixte' voice which is a combination of air conducted sound and bone conducted sound in your middle palate and and frequency.