I'm assuming you're seeing a speech pathologist? Singing is one of the steps to learning the female voice. Or at least it was for me. Practice will get you there. Its more about breath control when maintaining the upper registers. When you begin to run short of breath you will naturally tighten the muscles in your neck and begin to drop back into the lower registers. For me, it was learning to manage my breathing. After that, it's all about muscle memory and practice. It can be disheartening at first. The key is to realize that to get to where you want to be, you had to start somewhere. Another thought would be if your natural voice may not be able to reach the pitch you're trying to sing in. You may need to lower the frequency until you have that, then slowly train yourself to go higher. You'll have it down pat and soon forget how your male voice sounded. I'm sadly still working on the muscle memory part. I get terrible anxiety when speaking with people for the first time, or being put on the spot and I immediatly drop to my male range. It's reflex and 38 years of training to go deeper with my voice. On a good day when I'm relaxed, I zero issues speaking or singing in the upper registers. You'll get there in time. 🙂