Breathe a Sigh of Relief
Go ahead and breathe a sigh of relief, because it is time for something a little easier! Feminine voices generally sound softer and more breathy than masculine voices. Marilyn Monroe is an iconic example of this, as you can hear in this video.
When you try to speak in the upper range of your modal register without going into a falsetto, the natural tendency is to strain to reach those higher notes, which makes your voice sound harder, not softer, and not particularly feminine. This is because you put a lot of compression on your vocal folds, squeezing them together more tightly. You want to learn to use less compression for a softer sound, where your vocal folds stay open more (open quotient) while vibrating.
With high compression (closed quotient), adding breathiness will just result in a strained sound like Ash Ketchum from Pokemon, as in this video.
Learning to Control Compression
To learn to control the compression in your voice, start by watching this video and trying the "ah-ha" exercise and the vowel slides.
Then download the Android app Spectroid (or Spectrogram Pro on iOS), and in the audio settings, change the Desired transform interval to 10 ms (100 Hz) and check the box to Stay awake in the display settings.
Using the Spectrogram to See Your Open Quotient
With the app running, start by saying "ahh" for a few seconds in your normal speaking voice. In the scrolling display, you should see a bunch of bright yellow lines showing up against the purple and pink background noise. Then whisper "ahh" for a few seconds, just with your breath. You should see some faint pink smudges, but no yellow lines.
Now, heave a big, breathy sigh while saying "ahh..." in a soft, relaxed voice. Ideally, you will see faint yellow lines melding into a background of pink smudges. This is what it looks like when your voice has a high open quotient. It is somewhere in between a normal voice and a whisper.
Sliding Between Whisper and Voice
Your main exercise is to slide between a whisper and your normal speaking voice. Start with one long, whispered "ahh" that you gradually turn into a spoken "ahh" and then back to a whisper, just by changing the compression. Do this with the Spectroid app running, so you can see the change as well as hear it.
For a bit more of a challenge, try smoothly changing from a whisper, to a soft voice, to a normal voice while speaking or reading out loud. You can also play with this during your daily speaking practice. See if you can make your voice a little softer, or really breathy, or change from one extreme to another while still maintaining the feminine aspects of pitch and resonance that you have been working on.