Hi Bigmumma
Fortunately it hasn't happened to me, but it does unfortunately happen a lot. It has just happened to some friends of mine where the husband has a serious cancer that is currently in remission and his wife of thirty years has left him - can't cope with the cancer! Another friend had the same thing happen when he was diagnosed with cancer several years ago. So like you there are left with a double whammy, dealing with the cancer and feeling alone and let down by your partner. I don't know of any statistics but hear a lot of stories similar to yours.
Don't be afraid to seek counseling - use the cancer helpline 13 11 20. Don't be afraid to use resources available in the community - local counseling centres, or seeing a psychologist through your GP.
Take care
Sailor
Me howling blasts drive devious, tempest-tossed, / Sails ripped, seams opening wide, and compass lost. William Cowper