Yes, you need to relax. If you feel yourself tense up while riding, just take a deep breath. You will feel your whole body relax.
I think you need to carry your hands more and bend your elbows a little bit more. Also bring your shoulders back and look up. You have nice heals, but your leg is little too far forward. Bringing your stirrups up 2 holes (you're going to feel like a jockey until you get used to it) it should help with your leg. You should have a straight line from hip to heal, right now you don't.
In the jumping photo, you are jumping ahead, try sticking your butt out a little, and hover over the middle of the saddle. Also you need to bring your shoulders back so your back is more flat. Your back should be parallel with the horses back. Your leg looks OK, it slid back a little, nothing too drastic though and your heal is still down. Bringing your stirrups up will, once again, help with where your leg should be. Last, release over the jump more or have a loop in your reins. A short release is fine for this jump height, but your horse looks like he isn't able to use his body how he should be and he kind of seems uncomfortable.