Everyday Mindfulness

I am sorry to bombard you with another question on this and I am looking into getting a teacher, honest!

We're taught that, to be mindful, we should decentre by stepping back, acknowledging a thought and carrying on with what we are doing.

But how do we know whether to act on the thought now or in future, or if it is based in reality?

I'll use a crude hypothetical example. I'm doing something when I notice a new blemish on my skin. My initial thought is, "Oh God, not another one! [I have had things removed before, all okay, though]. I'm going to get skin cancer eventually, this might be it. You need to get this sorted ASAP."

How do I know whether that thought is valid and whether I DO need to get checked out if I notice it but don't accept it's content? If it is let go, will it arise again? If it arises in future, do I just do the same?

This is a thought about a serious matter, but what about a thought like, "My brother should help me more with my grandparents, it isn't fair on me?" How do I know if that is a thought with any validity or not?

If we practice mindfulness to its Nth degree, how do we know whether or not to do anything at all?

I am also confused about a passage in The Miracle of Mindfulness by TNH. He mentions being mindful as one is walking and talking with a friend. He says that one can do this, but if you have a thought saying "I wish he would shut up so I can concentrate," you are no longer mindful.

But, why not, if you are aware that you are having it, at the time you are having it? Or, why not, if you notice after you have had it that you have had it and you return to your informal practice?

No-one else seems to have these issues so the misunderstanding must be mine.

i would see them all as thoughts and let them all go.
when i meditate i do just that.
whether they're important or not isn't important at that time you can check the blemish out later and take action.
or you could use the blemish as a focus, notice it's shape size , colour, much the same as using a candle, flower, picture etc. but letting go of the thoughts that attach to it at the same time.
i've done that with random pains that crop up from time to time.
i use my meditation time for meditation, i try not to use it as a planning meeting with myself

Well,

Thi Nath Hanh is a Buddhist zen monk. While he has the same idea and concept of mindfulness as for example Jon Kabat Zinn his interpretation of the whole subject is not the same.

The theory behind our western mindfulness practice is profound ( and the best thing that has happened to me in my lifetime ) but it is shallow. It's only a very small set of generic ideas.
For zen buddhism mindfulness is a central theme but there is much more to TNH's vision of the world then just being mindful.

So there is a lot of feedback between his mindfulness practice and other meditational objective like compassion. Which he does in his very own way.

JKZ offers you a way to develop your very own and personal mindfulness practice by telling you "there is this technique, its basics work like this: ..." - after that you can frolick and develop yourself.

TNH's books are based on 2500 years of buddhism and he is presenting you his interpretation of those 2500 years plus his life long meditational practice. While I personally love his books and his gentleness is a very strong source of inspiration I don't feel obliged to follow his interpretations to the very letter.

EDIT: In your scenario the tnh way would be to be a compassionate listener. You should listen with your whole heart because that is how you ease the suffering, pain and longing of the one who is talking to you. If you think "shut up and go away" then it is not mindful anymore. TNH's mindfulness here is the idea of the lifelong focus on lessening the suffering in the world. This includes gardening the flower of love and compassion on the compost of your own anger and pain. So only if you have this thought without being aware of the thought and embrace it with your own compassion for yourself and your feelings you are not being mindful.

Having angry feelings or thoughts doesn't make you less mindful.
Being angry is a state with absence of mindfulness.

Also I think that though TNH's english is way better than mine a lot is lost in translation. His own native language is shaped by 2500 years of a non-dualistic religion so there are a lot of things you can say easily in vietnamese that are barely portable into languages shaped by dualistic traditions (i.e. everywhere that is not asia generally speaking).

JKZ's books are also based on 2500 years of Buddhism. Back in the late 70s it was his Zen teacher who advised him to "throw the Buddhism" out and develop mindfulness as a secular practice so as to be all-inclusive, which is what Jon was aiming at when developing his clinic.
Wherever You Go could be described as an instructional book but many of his other books (Full Catastrophe Living, Coming To Our Senses) are much more wide-ranging and could hardly be described as "a very small set of generic ideas". JKZ has plenty to say about compassion. His approach is commendably practical but never shallow.
In my view, JKZ and TNH are equally profound in their approach. Their teachings are there to be caught. It helps if you're ready. Though being open is probably enough.
Jon