I am looking for a word ...

Hand outstretched, Palm downwards, and shaking quickly from side to side. No words. Not sure I've described it very well though!

So so, tolerable, middling, not bad,

I like the term "muddling through" which implies to me that I am struggling a bit, but that is probably my own interpetation . A lot of people tend not to say what they are really feeling anyway

Mustn't grumble.

Apparently, this morning DH is 'ticking over'.

'Not too bad.'

Ca va? In French it works, even though I can't make my laptop do the bit under the C!

I can't bring myself to say I'm fine if I am anything but, so I say I'm all right. It satisfies the casual enquirer.

I have heard people say "You're seeing it".

There's a word in Welsh -"gweddol" -that describes what you mean, OP.

Muddling along.

Dunno how you are Cherrytree but I'm hunkytiddlydory-ish.

I think OK is the word actually

Canny means something different up here!

"Canny"

Different s...t, same day!

"Can't complain" or "Could be worse".

How about 'at ease' ?

7 / 10 !!

Equable?

I agree with fine. Short and sweet. Not overly exuberant or ecstatically joyful but I think it succinctly encapsulates fair to middling very well, but more briefly.
Don't like the "I'm good" thing. I think other languages do have phrases that translate along the lines of "things are going well", but in fewer words.

I think that "fair to middling" is perfect. Agreed it is not one word but you can't have everything in life. At least with "fair to middling" it leaves it open to the recipient to enquire further or not as they choose.

Are you perhaps overthinking this, Cherry?
If in answer to the question 'How are you?' I were to say 'I'm fine, thank you' I would mean that I was ok, not bursting with happiness but not unhappy either.
I certainly wouldn't be worrying that I couldn't find the right word to describe my state of mind.

I'd go for 'normal'

Average?

I am content with my lot, but worry can get in the way
I think fair to middling probably is where I am at the moment

I just feel we should have a word in the English language that sums up the bit between unhappy and happy.

Jalima your friend was right.
Happiness can last a short time maybe even just a few minutes or longer if your lucky, but it can also disappear in a flash.

Do any G/N linguists know if there is a word in another language that would fill the blank between unhappy and happy?

Granny23 my scots GP would say
'Aye I'm no so bad, how's yer sel?'
Not so far from what you said.

Maybe if you are Unhappy then you must be happy