I am so sorry for your loss.
No matter how ill our loved ones are and how well we know their suffering has ended we miss them in our lives.There is a void and it hurts.
Be gentle with yourself and know that time does help us adjust to our losses. There are many grief support groups if you check with your local hospital 's social service dept or community resource guide you should be able to locate one that wll offer the support of others who share your situation.
For many people it is difficult to know what to say to someone they know when that person has undergone a loss.
They want to help you feel better about it and say clumsy things like They are better off...it was for the best...or as in your case you will have to go on..these things are to some offensive and hurtful, though given with the best intentions usually.
I find it helpful to remember that however much I miss the ones I have lost, I could never wish them back in the suffering and pain of their former condition. I would never want to ask them to come back to endure a lesser life than they were used to for my benefit.
May time and Faith sustain you and heal your broken heart.
God Bless You and Yours.