It’s a recurring problem. I have only become Lucid 3 times but this always ruins it for me.
Every time i realized I’m dreaming in a dream i start floating mid air and i can’t stop it.
The most recent lucid dream i had i quickly looked at my hands and screamed lucidity x 1000 like the guide says but i found myself opening my eyes and waking up.
What do i do to prevent this?
Well from what I have researched one good way to keep a hold on your lucidity is to rub your hands together or spin around. If you find that you don’t have enough time to do so before waking up, your RC may cause a bit too much adrenaline or excitement causing you to lose lucidity almost instantly. The only way to fix that wound be to swap out with another RC.
I hope this helps!
I almost forgot!..When you spin don’t be alarmed if you appear in a new environment as this is recorded to be normal!
There’s always the edit button LucidLife 
Also, Yargo, when you dream, don’t be alarmed, period. You are completely safe in there, as dreams are a protected environment to allow you to experience and discover now things (among other stuff). Lucid Dreaming is the art of accepting what’s already in place, and changing something here and there that you care about; for example, if you were to explore a forest, you would not create the forest, just walk through it and maybe light yourself a magical fire somewhere inbetween.
So, the key to letting a dream continue would be staying put, and allowing whatever is to continue, with controlled exceptions. If you start floating, you can always see where it will take you 
spontaneous experiences often make for very interesting ones. If you really don’t, then first relax, and assure yourself that the dream is in your control, as you’re its owner, and then proceed in finding a way to pull yourself down, like imagining your increased weight.
Sweet dreaming 
You don’t need to prevent it. Just float.
Yep I agree, if you are floating that means you are a natural flier! Enjoy the feeling! Tell yourself I am lucid over and over and don’t stop. This really works for me. Sometimes when you say increase lucidity you have to say it a few times. In one of my lucid dreams I said increase lucidity, and the opposite happened. I just hollered, “I said increase, not decrease!” The dream got clear then. I think our ego messes with us occasionally, to make us want to give up. So keep trying and be firm with your desire to become lucid. 
It’s probably nothing to worry about. You will most likely have many lucid dreams with your feet firmly on the ground. Most of your NDs do not include any floating at all, right? Maybe you are thinking when you realize you are dreaming that there is nothing keeping you on the ground and it causes you to float. Well, what’s keeping you floating in the air? And what air is that, by the way? I’ve tried to not say “don’t do this”, but, instead of trying to “stop the floating”, maybe just try to “move down to the ground”.
I find trying to breath deeply helps.
About the waking up problem, I recommend to try not to open your eyes next time, and put your self in the dream again (imagine you’re getting into the dream again, like your zooming in or something), helped me last time I was lucid.
LOL Thanks for reminding me tosxyChor!