Take your time, go slow. While you are still Low play with your gear. Hang upside down in your saddle. Bounce on it. Swing out on your ropes, before you start up. Establish confidence in your equipment.
As you proceed up the tree, when you first start feeling a little nervous, before you get into full blown Fear, stop and hang out at that height until you feel comfortable there. Play on your ropes again. Swing out a little, roll in your saddle, work around the diameter of the tree, check all your rigging. When you start feeling a little bored, go on up a little further.
Don't feel like you Have to get all the way to the top in a tall tree when you start out. Take only as much as you feel comfortable with. I would advise you Not to push your threshold beyond nervous into fear. Panic is definitely a killer. If you are already scared you may be pretty close to the edge of panic. Might not take much to push you into it; a bee or wasp, unexpected resistance or slack in your friction hitch, etc. Go Slow. No hurry. Take YOUR time. They tell new motorcycle riders Ride Your Own Ride, don't try to "keep up" with a more experienced rider. I will adapt that advice to our sport, Climb Your Own Climb, don't get in a hurry.