The warming-up issue.
Many people think warm-up is just a simple walk, trot and canter around. Ride some circles and done. No, this is the part that gives you the chance to feel your horse. If it's stiif, wants to work with you today or if you have to ask for everything. Give atleast a 5 minutes walk with loose reins. Then ride round and low. Not Rollkur wise (Against that) lower and not as round. This loosens muscles up in your horses neck that are stiff. Then still in walk, ride some circles. Then him/her look outwards and inwards, without moving from the circle. Do some halt-walk transitions. Time for my favourite method, flexible movement. You tighten your reins (still riding on the bit) and give him more rein, repeat this about 3 times. Then do some small circles, loosen his neck up.
Do the same again. Now we go back to relaxation. Is he still stiff or leaning into your hands? Make a halt and press. What I mean with this is put your hands where the saddle is and keep pressure on the reins. The horse will want to loose that pressure and will go lower and eventually letting go of the bit. Try a slow walk, does he try and escape? Do the same thing again. Keep trying it! It will work.
Still having problems? PM me and tell me how he feels when riding. Which side is stiff? Any problems recently?