Kareyn, as others have said, it is hard to guess, but will give you some observations based on what I have seen others struggle with.
Firstly, if you come to karate quite flexible (or rapidly gain flexibility) your flexibility can outstrip your balance. You can end up throwing kicks that are higher than your technique and balance are ready for. Balance will come first at the lower kicks.
Secondly, ALWAYS keep the grounded leg slightly bent. The sensation you want is, as Big Al has mentioned, similar to the sanchin feeling, of something pulling you into the ground. If you can have someone pull down on your belt while you just try to do a hiza geri, you will start to get the feeling.
Another slightly more advanced exercise that helps with this is to get the mechanics of the kicking let going - so maybe do a few just with one leg, and then shift your consciousness into the grounded foot. Keep the kicking let going, but with each kick, focus on pushing the grounded foot into the floor. Almost like you are kicking with both feet at once. Change legs, and get it happening in the other leg. Then see if you can do it (slowly) alternating, then building speed. Kick the ground as you kick your opponent.
Tuck-extend-tuck-ground is the kicking mantra. The kick is not over until all 4 steps are complete. Doing this very slowly, and holding the extend out just for a one second pause will build balance and stamina.
Happy practising!