1. Eat more food, especially protein, so that you can increase your bodyweight.
2. Reduce your squat work-weight increment from 5 kg to 2.5 kg. The stronger become, the smaller your increments must become.
3. Remove lunges, split squats, and extensions from your program. You don't need them, if you're squatting and deadlifting.

In response to your "need help at the bottom point to do the remaining 2 reps": Don't do this. If someone else touches the barbell then it's not your rep, and you should not count it; otherwise, you're just fooling yourself, and you'll have no objective way to quantify your work.

1. Eat more food, especially protein, so that you can increase your bodyweight.
2. Reduce your squat work-weight increment from 5 kg to 2.5 kg. The stronger become, the smaller your increments must become.
3. Remove lunges, split squats, and extensions from your program. You don't need them, if you're squatting and deadlifting. Please consider the Starting Strength program, which can help you determine appropriate work-weights and frequencies for these exercises.

In response to your "need help at the bottom point to do the remaining 2 reps": Don't do this. If someone else touches the barbell then it's not your rep, and you should not count it; otherwise, you're just fooling yourself, and you'll have no objective way to quantify your work.