I am glad to hear that your lesson went well. I remembe getting bruised inner knees , too, when I was first learning. You don't want to be gripping TOO hard with your knee. Think of the weight going down through each leg and dripping off the side, down into your feet. I believe you should have some weight in your feet, but not jamming into them . It feels more like "petting" your stirrup with your foot with each rise; just a tad more downward pressure in them . If you grip too much with your thigh and knee I can guarantee you will lose your stirrups, especially at the canter.
As for trotting and posting, just go for short distances and when you feel that you need to grab the horn or you'll lose it and fall, then go back to walk. The amount of time you can stay in the trot will get longer and longer.
Believe me, posting is NOT natural and everyone struggles to learn it, but once you get it, it becomes totally natural and the easiest thing in the world. I can now post for a long time before becoming tired (well, not now 'cause my back is out).
It's fun to hear how much fun you are having. Be sure to have someone video you now so that in a year you can wathch the video and get a good laugh.