When I tried it to learn, the only problem was what you said. And I solved that problem with this. When you do crossstep, you're supposed to cross your legs. I try to find and keep the balance while crossing the legs.!! Just cross your legs on the moving board and try to find the balance then you'll get it.

And also hanging(?)your arms in the air helps your balance

If you practise skating switch ( on your wrong leg ), you will have a lot better balance . If you can keep your valence with only your wrong leg, you will find it easy.

if you use your arms to even out the balance ive found that it helps alot

I practiced holding on to a chair but the board has to be on concrete. That way I know how my weight moves around as i do the trick but i avoid falling. both feet are facing at an angle instead of perpendicular to the board. My front foot is also slightly hanging off the board (makes it easier to cross). To keep my balance, most of my weight is on my heels.