In the last 6 months I've taught my horse to work in a contact from scratch. He does in halt and walk without me even asking, and once he gets his head down in canter he will keep it for as long as l want but l he finds trot really difficult. l have to work him for about 45 mins before he relaxes and keeps in it the trot, otherwise his head is constantly up and down. Does any one else find trot the hardest? Any tips?

This is because he is not supple at the trot, you need to get it there first before you can get it at the other paces. This shows he is not coming through, using his back properly, and rounding. Its probably due to you having stiff hands that dont give. Go back to dressage basics, all your doing is keeping your horse in a long low false frame.

l know what you mean! My pony was to learn that too. My sister is helping me teach her. l would work on the walk/halt till you have that down, (sounds like you do) but l would keep working on the trot until you have that down before you move onto the canter. Make sure you take on the reins, but have steady hands, and be ready to 'give' a bit when he accepts it.
Good luck!

Canter definitely.. the trot is a-ok but the walk tends to get a little sluggish if l do not urge him on.

It is not just about putting the horses head down. You want them to lift their back along with their head being flexed at the poll.

Let him stretch at first, in order for a horse to contract any muscle it must first be relaxed. If the muscles is tense then it can not contract. So letting him stretch for a little bit will loosen his muscles and then you can ask him to go on the bit.

Okay 1st things 1st. What it sounds like is that u have him in a false frame, just his head is in the correct position but he is not really coming over his back, or he is not really round & on the bit. Going on the bit comes from forward, not just having the head in a certain position. What l usually do with horses that r younger, l usually ride with my hands down very low on either sides of their withers & close to their shoulders (but not so far down u r getting close to a two point) almost like draw reins. This encourages them to come down over their back & really stretch into the contact. And when they pop their heads up, only wiggle with ur fingers & wrists, not ur whole arm. Do this with every gate. Also when u r doing this, u should be asking for more forward, in walk, trot, & canter. Once it seems to be the case that ur horse is taking to it very well (it sometimes takes over a week so u have to be patient) gradually bring ur hands up to their original position & if ur horse pops his head back up, put ur hands back down. Eventually he should get it! The only thing that is hard about this is that u tend to get the bad vice of looking down, so make sure to keep ur head up! Good luck!