First off, I'm glad you're OK. I want to thank you for sharing your "screw up" with us. I've made mistakes and shared them too. It's a good way to learn from each other.
From what I understand, you're mistake was to use non-climbing rope and an overhand knot for your prusik loop instead of climbing rope with a double-fisherman knot. I never considered what you did would be dangerous but now I can see why!
As I don't have a proper saddle and use a home made "ass sling" in conjunction with a rock climbing harness, I'm always connected to my tether in two spots. I use a prusik made with climbing accessory rope and double fisherman knots to connect my "ass sling" above a ropeman connected to my harness. When I get situated in the tree I let my weight transfer into my sling until I'm comfy.