I would say your sail handling is not good yet. Sometimes you are sailing sheeted out (spilling wind off back of sail) and sometimes oversheeted (sail too much across the wind).
You can practice sail handling on dry land if you have somewhere to set the board that wont damage the underside. You want to practice positioning your hands so you are getting the same pull on both arms and learning to lean back against the pull of the sail. 90% of windsurfing is sail handling.
When sailing, you want to be confident that you can control the power of the sail, so on dry land, you would practice turning the power of the sail off and on to gradually be able to lean further back against the pull and how to bend your legs to adjust for sudden lulls in the wind.
You can do the same thing on the water, but its much faster to learn it on land.
The guy in this video is a couple of stages further on than you, but it gives a good idea of where you are trying to get to.
He is still a beginner so he is not doing everything exactly right (is maybe getting too much pull on his back hand), but his stance and hand positions are good enough.