Trust Your Horse, Trust Yourself
You can't ask your horse to trust you if you aren't trusting them.
If you're standing on the mounting block looking at the saddle, and apprehension and nerves are swelling up inside you, you need to listen to that just as much as you should listen if it were your horse's feelings. If you had a relaxed demeanour but your horse was moving around anxiously, wound up like a tight spring, you wouldn't just get on (or at least, you really shouldn't). You would go back several steps until before he was uncomfortable and you would work on every little step from there to pin-point the problem and be able to work it out. By the time you're mounting up, your horse is standing calmly and relaxed at the mounting block with a loose rein as you mount.
But what about you?
I usually make analogies between horse training and training focussed at kids or other people, because it helps you understand what im saying from a more familiar perspective. However with this situation, it's easier to compare what *you* need to work on for yourself, with horse training.
Think of it this way, repairing your own trauma or insecurities is no different from repairing them in a horse. You essentially need to 'train' yourself, one baby step at a time. To do this, do for yourself exactly what you would do for your horse. Liberty ground work and establishing a 'language' based on positive reinforcement techniques is an ideal place to start. Work on simple things together first, haltering, walking at liberty, stopping together, picking up hooves, grooming, and eventually teach your horse to be saddled up while completely relaxed, (these things may seem like they're training the horse and not yourself, but believe me, they are training you just as well). Every tiny step not only increases the horse's trust and confidence in you, but also increases the trust and confidence you have towards your horse. If something creates nervousness in either party, take a step back and return to that place of relaxation, before finding a smaller step forward.
After all of this, when you're standing at the mounting block and your horse is standing relaxed and confident in you, instead of apprehension, you will feel nothing but that trust and connection between you.
Only once that trust goes both ways will everything really just "work".