If it's too expensive to be on your partner's insurance as a learner driver, you could rent a car using your US license (since you've been here less than a year) to practise. Maybe a weekend or a day when your partner is off so you have someone to help you.
Also, if you're not confident with your lessons, consider trying another instructor. DH is always stumped when his coworkers and friends complain about their instructors, so he urged me to shop around.
I had a lesson with one instructor who wasn't a good fit and not experienced at all in teaching international drivers. I tried another instructor and he was perfect! I felt confident and passed my first try with only two minors even though I was a wreck after my lessons early on.
It can be hard to adjust to driving in the UK, but it might not be your fault.
