My friend was new to riding and bought her first horse about 2 years ago. She wanted so badly to lope / canter but her mare wouldn't do it. Bottom line, the mare knew the rider wasn't ready and was protecting her from getting hurt. Flash forward 1 year.......(yes, an entire year)....my friend is happily loping and her mare is a very willing partner.
You don't run before you learn how to stand, then walk. Take your time, learn the correct fundamentals of balance, a proper seat, and proper cues before you add speed to the mix. You have many years to ride and it is well worth it it to learn slowly and correctly.