I see many riders at the lower levels actively holding their horses back (either deliberately or unconsciously) and not allowing them forward. They are riding with the brakes on. It doesnt make for good balance and rythmn which are the foundation of effective cross country riding. Speed, when one is feeling nervous or uncertain, can feel a lot faster than it actually is. A useful exercise for both you and your horse might be to practice riding at speed. From a starting point on a track or in a field, measure the number of metres per minute appropriate for the level you are riding at. At BE80, the UK beginner level, you should be cantering at 435m per minute so clearly mark 435 m from your starting point. Then simply ride your horse at a comfortable speed while timing a minute. When the minute alarm sounds, see where you are in relation to your 435m marker. Too slow, then push the canter on a bit, too fast … Practice frquently until the right canter speed becomes very familiar (and it will vary from horse to horse). It helps build confidence, control and rthymn in both of you. When you are secure at speed the jumps become less of a drama and more of a dance.
The other useful tip is to instil a really good halt. Practice walk to halt, trot to halt and then canter to halt using body position, seat and weight rather than reins. If you watch top riders running xc their hands are really still and it is body position that rates speed. If you trust your horse to stop on demand then it is easier to ride xc with confidence.
Like any jumping, it is all in the flat work between obstacles.
