Welcome to the world of heartaches and heartbreaks. I have been there. It hurts a lot more when they leave without a trace and more when they give us no reason.
You were cheated. No doubt about that. Acknowledge this first. You are the victim here and to imagine you are going through all this along with your cancer is devastating. I pray to God that your pain lessens as soon as possible. The first thing that you must do right now is to seek answers to the questions that are haunting you. Getting convinced is the only way out. Trust me, it took me more than 5 years to get the answers I sought when I was going through a heartbreak. So the sooner you find answers to these the better it is for you. I can help you here – only if you tell me what those questions are. Another way is to pray and communicate with your God. Write letters to God – start journaling. Let the pain flow out of your body. Drink more water to cleanse yourself. Develop self-love. Seeking love in another person always leads to pain. Hence we should love ourselves more than we love anyone else.

>>I’m still so heartbroken over our breakup
There are 2 ways of handling this:
1) Self-healing – Practice confiding in your God daily, every now and then. Start journaling. A quick 5 min meditation / transcendental meditation will help.
2) Take healing – from a professional – psychologist or a life coach. Take therapy.

>>and it’s really hard to know how easily he moved on from our relationship.
For all we know, it is you who thinks that he has easily moved on. He might be having his own inner battles and suffering from inner chaos.

>>I feel so stuck and want to move on but I’m really having a hard time doing so and seeing him this week feels like such a set back.
Feeling stuck is natural. Karma taught us that we need to be in it for some time until we learn our lessons. So I suggest you give yourself time with patience. Settle in the space and let time and nature take their turn. Seeing him will feel like a setback but let it not bother you. You have much better stuff to focus on – like your career and hobbies.

>>Also he told me that we wants to be friends and I just cannot do that after everything that has happened.
If being friends with him makes you feel like it is pulling you back, then don’t be friends with him. Nothing wrong with not being friends with your ex. It just denotes that you are comfortable on your own and laying the pitch to move forward in life.

>>It’s really hard to let go and move on. I still think about him (more than I want to).
Letting go is the last step. Some cannot really completely let go. Move forward if you cannot move on. Getting thoughts about him or thinking about him is natural. You were in a relationship for so long. Unless you find your purpose in life and derive goals toward it, you will be haunted. Learn to forgive yourself first. I am not at all telling you to forgive him. What he did is wrong. Totally wrong. But you need to stop beating yourself for his mistake.

Learn to spend some time with yourself. Pamper yourself. Hit the spa. Get a massage. Make God your best friend. He is the only one that understands you like no one else. By investing in yourself, you will heal gradually. And you will feel like this relationship was nothing compared to the purpose of your life. This is just a stepping stone. This is not everything.