STOP IT!
YOU'LL HURT YOUR THROAT!
YOU ALREADY HAVE!
Let your voice rest.
Developing a female voice from voice apparatus that underwent a testosterone puberty takes 6 months to a year for the majority of MtFs. You have plenty of time, do it right. And it's better to do several short sessions during the day/week than, say, one long session a day. My car is my practice area, for example.
BTW, your metaphor is accurate. What you have to do is get the viola body to resonate like the violin by minimising the resonances that are viola-like and enhancing the resonance of the parts of the viola that can sound violin-like.
Look for three voice feminization programs:
Andrea James' Finding your Female voice
Melanie Phillips' How to Develop a Female Voice
Exceptional Voice's Fundamentals of your Feminine Voice
Of the three, I think the last one best teaches you how to produce the proper resonance.
You may be 'blessed' with the skill/intuition/equipment/luck to find the right combination that works on your own, or you may need a guide to show you the road. Even if the former, formally learning how to produce the right resonance and formally practicing these techniques will pay big dividends down the road.
Make haste more slowly!
Karen