I just think that Steven couldn't live without the gem because he's lived his whole young life with it and the benefits that came from even being a half-gem/diamond, so suddenly becoming completely human took a toll on his entire being
Oh yeah, and the part where he forcefully had his gem removed from his fleshy human torso, which in any realistic situation would have left a giant wound in Steven's body, and thus would have been losing a lot of blood (hence why he was turning pale and weak), but since this is a kids show, obviously they wouldn't have been able to show such graphical imagery, but they still did that just to make the scene more dramatic
