Don't compare your riding ability to others. Everyone learns at a different place, and don't try to compare your skill level to those younger than you, who might be at your level or higher. Don't try to push levels, just go at what you feel comfortable with and know that not everyone is capable of reaching the highest levels, and it only matters if you're having fun. Don't make it so competitive that you aren't seeing the fun in it anymore.
That being said, know what you can handle. Don't go over your head in discipline or in the horse.
Never give up, and if you take a tumble get back on the horse immediately to both discipline the horse (if he's done something wrong) and get over that initial fear. The sooner you get back on the better chance you'll have of recovering.
Always remain active. The walk is the hardest gait to master because it is hard to have the right engagement and activity in the horse. Don't slouch and let the horse carry you, always /ride/ and control the horse, and remain actively cueing, moving, and looking around in the direction you're going.
Try not to ride tense. I found that when I am relaxed my horse is relaxed.
Also, I go back to the basics once in a while and just practice the easy stuff. Here is my list of the very, very basic stuff my trainer has me go back to:
Heel down, toe in, hip rotated in, sit up stright, look up and where you're going, not down.
My most important tip: Don't overthink. Do what feels right, and trust your trainer (I'm assuming you're taking lessons? If not, whoever you ride with.) Trust your instincts and ALWAYS go with your gut.