I keep trying to contact my ex but he will not respond
This is often a difficult and most painful sign that your ex wants some space.
The first mistake you are making is trying to call him when he is not ready to talk to you. He needs time to think about what it is like to be without you and miss you. If you keep trying to contact him all he will think about is avoiding you. Not about how much he misses you. It will be imprinted in his brain that you are needy and desperate and all he will think about instead of missing you, is how to avoid you.
The problem is that you feel you have been doing the right things to make this relationship work but in fact, you have been making a few wrong moves that were making him feel suffocated. This is often why guys distance themselves after getting into what you thought was a committed relationship. He may have thought he wanted a relationship with you as well, but felt you "changed" when suddenly you were calling him instead of waiting for him to call you.
OK, this relationship may still be salvageable, but you must back off from him for awhile and change your tactic. Give him some space. I know you are hurting, but to let him know this is only going to make him feel guilty and the worse he feels the more he will want to stay away from you.
He may care for you, but the actions you have been taking are that of the pursuer and that puts him in the feminine role of the one being pursued, which men do not like.
It is going to take time and a change in tactic, if it is to be fixed. The whole dynamic of this relationship, the way in which you relate to him, will have to change for this to work and it will take some time before he is ready to communicate with you again.
I would start with giving him some space to miss you. The second thing is do the best you can, to keep yourself busy filling in the time you are thinking about contact him, with something else in your life. You will need to fill in the empty space he has left in your life the best you can for now.
You can rebuild a relationship with him later, but if he is avoiding you now and you keep chasing him, you will lessen your chances of having any relationship with him in the future.
Yes, there is a possibility that there is another woman involved and that may be he is avoiding you out of guilt. A psychic reading can help you determine if this is the case..