I went to a class for my Crown Jewel that was 4 days long. It covered all aspects of the machine and frame. It included lots of practice and discussion on different patterns. I came away confident. I still needed practice but the stress was gone as long as I treated myself gently - giving time to become experienced.
I think you need to find someone who can work with you for several sessions - even another longarmer who has a different machine and frame who you can pay to work with you on tension, loading, thread, needles, sewing straight lines, sewing curved patterns, using longarm rulers, pantographs, etc. Then come home and make some muslin sandwiches and just repeatedly sew patterns.