I agree with all those who have said that my advice is easier said than done. I disagree however, with those that have said to have a trainer do it for you. In my experience, your relationship with this horse will not change for the better until you can get the horse to do it for you. Don't get me wrong, a trainer has value, especially in teaching you safety and how to work with the horse. But you need to do it. If your trainer/instructor can't teach you to lunge this horse- Well, I'll reserve comment except to say there are good trainers out there that can help you.
I hope you noticed I said, "get the horse to do it for you". I think there is a real difference between having the confidence, assertiveness and self-esteem to get the horse to do it for you and being aggressive and forcing the horse to do it. Both will change your relationship with the horse, but only one will improve it.
Unfortunately I have a hard time explaining the difference between the two.
I worked for two years as a horsemanship instructor at a residential treatment/detention facility for troubled youth, then over 7 years as the horse program manager. I taught over 450 kids to ride and work with horses. Whenever a kid was having trouble with a horse and I would tell them to get more assertive they would often think I meant to get aggressive and/or mean. That is not it at all. But when the kid would develop the mindset of "it will happen and I can do it" often the problem would be over before he/she crawled back into the pen with the horse.
I used to think that a horse could smell fear, or for that matter confidence. I don't totally discount that idea, but I now think that a horse is so sensitive to non-verbal communication that he can tell when you are serious about something. I think posture, how you hold equipment, how you move, speed of movement, facial expression and even the diameter of your eye (among many other things) tells the horse if you can accomplish a goal.
I have been pretty **** nervous in a round pen with a certain few horses. If they totally worked on the smell of fear, I would have been toast. But I have habits, experience and confidence that got me through it and I accomplished what I needed to. I guess I am telling you that you can "fake it".
When the kids would say they didn't have the confidence to do something I'd tell them to pretend they did until they developed it.
This is not just a 'cowboy up' talk. I think when people are encouraged to cowboy up, they are often being invited to do something stupid. Be smart! Be safe! Be confident! Develop an attitude! Learn what you need to do. I believe you can. Keep us posted.