"The woman has a strawberry."
I tend to speak quietly when I'm practicing, so I can't do it either, but when I say it loudly I can, because there's more force to it which makes it more effective. It's the air passing over the tip of your tongue making it vibrate against the front of the roof of your mouth that does it. I don't have trouble with "fr" so much as "tr", though. If you can't make the sound at all... Maybe practice going "PRRRRRRRRRRRRRRRRRR!" really loudly and high-pitched, with your tongue how I described before. I tend to find "pr" or "br" the easiest to start with. Good luck! :)
Your "R" shouldn't come from the top of your mouth, it should be from the place right behind where you pronounce the letter "T" and the tip of your tongue has to be barely touching it, also your tongue has to be flat so that it closes the top of your mouth entirely from the bottom, when air comes from your throat forcing your tongue to vibrate it will make that sound.