I am new to MTB’ing (going on about 6 months now) and I was having the same problem, expecially when I went clipless (would never go back now though). Let me say thank you for the advice even though I did not post the original thread, but I will benefit from this also. I do have to say that part of it too will be time in the saddle with familiar trails. Anyone, however long they have ridden, will run into problems on a brand new trail (to them at least), especially if they don’t have a buddy who knows the trail or a guide of some sort. Continue riding your "home trail(s)" and as you get to know them, you will know when to shift. And with that, you gain more skill AND confidence, which both can carry over to the "new" trails in the future. I would get frustrated when I tried a new trail but I had to realize I had no clue what was coming. Cardio work and weight training will certainly help, but not always. Some hills are just going to creep up on you.