I need help with seeing distances?

The other horses you ride sometimes probably have a faster rhythm or a different stride length that makes it easier to find the distance on. What you really need to work on is pushing your guy into a different stride length over poles placed at different points in the arena and smaller jumps so that you can really focus on getting a better eye. The reason you get it 50/50 (in the video more like 25/75) is because you aren't adjusting pace at all. Your horse's pace was too slow in the video which made it so that the stride lengths were too small for the correct placement over the fence. Utilizing half halts, your leg, and even staying more in a two point position will help with getting him placed in the correct spot, rather than so shallow. If you find that you keep ending up shallow and deep, the pace is too slow, and if you keep ending up wide and long then your pace is too fast. The only way to correct it is to correct the pace by practicing lengthenings and shortenings of stride, preferably over ground poles.