You make a perfectly valid point captured by Juvenal's question from long ago :
Sed Quis Custodet Custodies? [But who is to guard the guards themselves] (Juvenal, c. AD 60-130)
Any familiarity with state-based politics should realise the sharpness of this question. Once you have a political group or person (president, monarch, parliament, Congress) separate from the governed, then that person or group can readily acquire interests of their own that either are different from those of the governed or even antagonistic to them. The great Italian theorists, Mosca, Pareto and Michels, none of them remotely an anarchist, made just this point, as did the economist, Joseph Schumpeter, who saw political groups as 'entrepreneurs' buying votes by selling popular policies in the their own interests of gaining or keeping power.
1 The claim that 'people are not capable enough/ not to be trusted to govern and take care of themselves' [without the present of supreme coercive power] is vacuous out of context. It depends on the group.
2 A standard method of 'guarding the guards' is constitutionalism, historically perhaps most clearly exemplified in the US constitution with its separation of powers and system of checks and balances. Constitutionalism is certainly not without its value but its success depends on who operates the system : and it is useless when the separate powers collude and when checks and balances are not applied.
3 What critics of anarchism usually totally miss - I am not myself an anarchist - is that there is no need for a sudden, total transformation of society along anarchist lines. That probably would lead to disaster of many kinds. But gradualism, a process of political education and experience is possible in which people gradually learn (probably in small but widening groups) how to become co-operating or at least responsible anarchist citizens. Recall Rousseau's Social Contract (1763) when he said that a self-governing citizenry would need guidance in learning how to participate in a political society operating under the conditions of the general will (volonté générale). They would need to acquire, and should not be assumed unrealistically to have already, the necessary dispositions and skills. A Legislator would be needed (but how to find him or her ?) but would hold no political office. The Legislator works purely by persuasion and does not hold coercive power of any kind.
J.-J. Rousseau, The Social Contract, 1763, II.7.
Constitutionalism: Nomos XX, Published by New York University Press (1979)
ISBN 10: 0814765734 ISBN 13: 9780814765739
Vile Mjc, Constitutionalism And The Separation Of Powers. Published by Clarendon Press Reprint, 1969.
Mosca, Pareto & Michels : see W.G. Runciman, Social Science and Political Theory (New York: Cambridge University Press, 1963.)
Coe, Richard D. & Wilber, Charles K. (editors), Capitalism And Democracy: Schumpter Revisited. ISBN 10: 0268007519 / ISBN 13: 9780268007515
Published by University of Notre Dame Press, Notre Dame, 1985.
Jon Purkis, James Bowen, Twenty-first Century Anarchism: Unorthodox Ideas for the New Millennium (Global Issues), ISBN 10: 0304337439 / ISBN 13: 9780304337439
Published by Continuum International Publishi, 1997.