30 minutes with full focus, I think, is more than enough. You will feel when it is okay to stop because you are energy-sensitive, so you can sense that everything feels lighter. Dark magic has a heavy consistency, which is why many people can experience shortness of breath. Your reaction to the field sounds like you really have dark ancestors. I hope you feel better every day. Skip a few days for meditation because your body needs to regenerate.
You can meditate with a few fields at the same time by focusing your attention on mandalas and some people (me included) visualize energy. With TB you can visualize points in the past by bringing those memories back and change them or instruct fields to work in this timeframe.
plenty of ideas here