As long as you do not start to get really freaked out after passing the 25 ft mark, you can defintiley condition yourself to go higher and higher, and be increasingly comfortable working at greater and greater heights.
It comes with more and more time in the tree, and gaining an integral trust of your gear.
This is where buying some of your own gear can be a real boon to your confidence. Trust me in that it is a way better feeling strapping on your own belt and spurs that you take care of, and chose too purchase 'cause you liked them or whatever your reasons, all that and you don't share them with climbers who might no share your same sense of care. Same with rope, INSPECT INSPECT INSPECT, first thing in the morning I hand lay my rope, using my eyes and hands to inspect the rope at the start of the day, and again at the end, learn to keep a good eye on your ropes, take care of them and they will return the favor.
Another rule of thumb a GREAT old climber told me a ways back for dealing with getting freaked out in a tree, slow down and take the tree ten feet at a time, dont think of it as a 70' climb but a series of smaller climbs , take ten-fifteen feet at a time, and before you know it your at the 70' mark.
If there comes a time when you do get a bit freaked, and it happens to all of us at one time or another, again just trust me on this, guys who say they have never been scared, or don't get scared ever, they are either lying, or too foolhardy too survive long in this game.
Fear is a natural part of the job, learning to deal with it in a calm and rational manner will not only make you a better climber, able to work in a safe and controlled manner, but it will also show up any time you are in a panic situation.
Anyways back to when you do get freaked out, first stop your mental process as best you can, slow your heart down as best you can, a series of slow controlled breaths, it can be tough at first, but calm down as best you can and assess the situation, then you come up with a plan of action, be it simply moving 5-10 feet more up the tree, whatever it is and then you move ahead with your plan.
When you reach your destined mark, you repeat, calm, assess, execute.
This is the way I was taught to deal with fear in climbing and it has served me well.
Good luck Eddie, and stay safe,
-Grais.