Unfailing means never running out and never letting you down — especially when things are hard. Notice that the difficulty is part of the meaning. Anyone can be kind on an easy day; 'unfailing kindness' is kindness that was still there when the person had every reason to lose it. That is why the word feels warm and a little admiring. It goes before a noun, almost always, and it pairs with a small set of good qualities: unfailing kindness, unfailing patience, unfailing courtesy, unfailing good humour, unfailing support, unfailing energy. If you learn those pairs you have learnt most of what the word does. It is also used for things that always work: 'an unfailing supply of water', 'an unfailing remedy', 'an unfailing sense of direction'. Here it means reliable to the point of never once failing. The word is a bit formal and a bit old-fashioned — it belongs in thank-you speeches, book dedications, obituaries, and the acknowledgements page ('I thank my wife for her unfailing support'). In everyday speech a friend would say 'she's always there for me'. Neither is better; they live in different rooms. One honest note: because it appears so often in those set thank-you phrases, native speakers sometimes use it half-automatically, without weighing it. Used deliberately, though, it is high praise.