Welcome to aikido. Hang in there, all that is required is perseverance. It will all come with time.
Sitting on one's knees is called "seiza". Just practice it some every day. Start with 15 seconds and work your way up. Use a kitchen timer or a timer on a digital watch.
Dizziness during front and back rolls is normal. You'll get used to it, but it might take 6 months. Then, you'll only be dizzy after you stop