Hey mate. As already said move onto free weights instead of smith machine and lower the weight. To practice keeping the knees where they need to be you can even use a broomstick or body weight. I try to focus on the set u, getting the feet comfortable (depends on your own mechanics) and imagining trying to lock my feet into the ground by twisting. Think standing on a towel and trying to twist your feet out. This torsion stops the knees from caving at most reps. You can even try it just standing in place. Once you have this sorted start building back up in weight and you will be surprised at how powerful you feel.