I think it's very common, Patrick and has happened to me. Antidepressants help. Therapy is very useful too. Most hospitals or Cancer Societies have counselling facilities.

After finishing treatment there can be a sort of anti-climax where everything hits you and other people, so concerned about you before, have gone on with their lives. There's also a sense of being on your own after all the constant medical attention during therapy.

What you are feeling is oh so normal so take whatever help you can get:)

1996, ovarian cancer surgery + cisplatin and taxol.
September, 2007, SCC of left lateral tongue. Excision.
October, 2009 recurrence in scar tissue, T1NOMO. Free flap surgery from left wrist - neck dissection. 63 year old New Zealander. No chemo, no RT.
February, 2014. New primary in left buccal mucosa. Marginal mandibulectomy, neck dissection, right arm free forearm flap. T1N0M0 but third occurrence and some areas of concern: RT started 8 April and finished 19 May.