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Equilibrium issues with understanding watch

1. I can't get my head around equilibrium or more specifically, the 'equilibrium shift'.

Suppose we have:
A + C -><- C + D

the principle states that if we add some A, there will be more products formed, so ultimately the amount of A will decrease and the amount of C and D will increase. This makes the equilibrium shift to the right.
Well, I can't help myself but take the idea further - if the equilibrium shifts to the right and the amount of C and D increases, surely the reaction will counter-act that, so that when there's more C and D, they react more and so in turn - their concentration decreases?

More clearly:
1. Concentration of A is increased
2. A and C react to produce more C + D (equilibrium shifts to the right)
3. C + D react (because there is more of it), forming A + C
4. Equilibrium shifts back to the left?

From my understanding, the equilibrium would just shift back to its initial position so that the concentrations are back to 50/50. A bit like on this diagram:

Where is the fault in my understanding? I am aware that something's not working out, whether it's my definition of equilibrium or equilibrium shifting. Or perhaps I am understanding it, just not connecting the dots with the information from the textbook?
I've been sitting here looking throughout the internet for some answers, but it seems that I can't comprehend the concept myself. Help please?

Thanks in advance.
2. Do some research on a thing called Kc and get back to us if you don't get it.

If A + C <-> C + D, then nothing appears to be happening to C, so the equilibrium is just A <-> D. In which case, your understanding is fine.
3. As already mentioned, eqm constant is the missing component in bsic level understanding.

What is worth noting also is that while overall amount at eqm can change after a "disturbance"/"perturbation", the ratio of conc of etc....ie eqm constant..remain fixed as long as T is constant.

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