I'm confused. Do you have a legitimate medical issue that has left you flat on your back in bed, or is this all just fear and you climbed into bed on your own?
I will say, the doctor you consulted sounds like a quack. On earth, we are all subject to gravity at all times, whether standing up or lying down. There should be no reason you aren't "used to" gravity. Rather, your muscles have atrophied through lack of use. When you stand, yes, you are going to feel shaky. ThAt's normal. The best thing you can do is get up and walk. Just do it multiple times a day. I imagine getting out of bed and physical activity will make you feel ten thousand times better. I always feel like absolute crap with worse aches and pains when I laze around in bed. I can't imagine how you feel if you've been there since February!!! Please just try to stand up, like right now!