First of all, you two are not even close to collection. Collection is something that takes a very long time to develop, and is not just tucking the horses head in. I would recommend following the training scale which is what truly leads to collection- rhythm, relaxation, contact, impulsion, straightness, and finally collection. Before developing relaxation, you develop rhythm, relaxation before contact, etc.
Rythm means two things - having an even tempo and being forward to the leg. Relaxation is when the horse is relaxed. His back is swinging with legs reaching under him. Contact is keeping a soft, steady rein contact that your horse is accepting and reaching into.
I think you guys are at relaxation. Your horse is hollow and stiff all throughout the topline. Like someone else said, I would recommend doing lots of long and low work. Thismeans lots of free walk and stretchy trot, getting your horse reaching down into the contact. For the free walk you don't need to have any contact, ideally you are at the buckle of the reins with the horses head lowered (around wither height but depends greatly on horse), he is walking freely, back swinging, and hind legs swinging under him. Stretchy trot is similar, but you have slight contact, and it is at the trot.
Please, work with a good dressage trainer. You have much to improve with your position, and you are doing your horse more bad than good riding with contact. I would go more into detail, but I am on my phone and have to go. Good luck!
“Good things come to those who wait… greater things come to those who get off their ass and do anything to make it happen.” - Unknown