Swimming in the air is always a good stepping stone if you are struggling to stay in the air. That fear you feel once you increase your altitude is most likely just a fear of the unknown. It is very common and I find it happens with diving into deep water as well. I am an experienced LDer and I still feel the anxiety from flying too high every now and then. Just reassure yourself that you are dreaming and nothing can harm you. You are the master and the fear of the unknown isn't going to stop you from exploring. Good luck to you!