First off, I don't think a hand mixer is an adequate tool to make a good pizza dough. It might work well for batters such as pancakes or light cakes, but for heavier doughs such as pizza or bread doughs, even if you'd use the dough hooks, I don't think it will cut it. I can't imagine a hand mixer developing a good dough structure.
I started out with a wooden spoon and the hand kneading technique but discovered quickly that it won't do the trick unless you have experience. And a lot of it.
The other thing is, when you say it's sticky...what do you mean by that?
Is it sticky in terms of it sticks to the surface of where you're kneading the dough? If it sticks to your hands, use some bench flour and work the dough vigorously, adding a bit of flour each time you notice that it'll stick again. However, it's a fine line you walking there, or all of us for that matter, when adding bench flour. Too much and it can ruin the outcome, too little and you're screwed, too and end up with a less desirable pizza.
Another thing is that your flour is not soaking up the water properly, resulting in a sweaty dough. You said you put it in the fridge after kneading and forming it into a ball. I'd let it sit on the bench or counter for at least an hour, covered with a damp towel, then reshaping it and putting it into a bowl. Then it goes into the fridge for a few days.
Usually, when I take the doughs out of the fridge after w few days, they are quite "wet" to the touch but a few minutes of kneading and reshaping does the trick.
Another thing is the age of your flour. Is it a fresh flour, something you bought recently or something you had in your pantry for some time?
Lots of questions, I know, but they will help us to help you