You have to mourn the loss of the relationship in which you had such high hopes, but you should also celebrate the beauty and happiness it brought to you as well. You said 'you want to start feeling like you can move on' well so does everyone else in a situation like this, but you won't feel like that until you have properly put this to bed in your head. Stop mourning what could have been and accept that there is something else for you.
Look at it like this, he had enough love/compassion/respect for you to not continue to waste your time. So you at least have a good man radar. He either knows he is not the right person for you or is unsure and really needs space to find that out, but either way he gave you the gifts of honesty and respect by not taking the relationship further or to another level in spite of his doubts. We look for relationships to be permamnent and then when they aren't we act like we have been wronged or robbed of something. In reality we have been given something...the chance to still find the thing we were seeking from the very beginning. He has given you a gift. He was good to you while you were together and he was good to you when he realized it wasn't right.
It sounds like the relationship itself was great as well. So look at it for the gifts it/he gave you, carry them with you and continue to look for the right man for you.
Obviously you feel rejected...which is understandable, but rejection is not always a bad thing. Especially when the thing we want so badly isn't good/right for us. You are worth more than being with someone who doesn't want to be with you and is too afraid to communicate that to you. SO be thankful that he did that for you and that you can start over with someone else.
Has he told you why he wanted to leave other than what you mentioned above? If not you can always ask. Don't feel bad or surprised if he can't give you the answer your looking for, but it might help you work through the whole thing. Also depending on how sick he was, maybe he realized he wasn't happy or using his life the way he should so he decided to free both of you from the inevitability of the relationship ending w/o wasting any more time.
All in all, you just need to know and believe that you will find the right man eventually, and when you do not only has this person helped teach you what you want from your lover, but freed you so you still have time to enjoy it...mutually.
Stay strong girlie.