Is it permissible to retract a vow made to Allah not to exceed one and a half bowls of water for ghusl (ritual bath), or a vow made to my mother to use soap for every ghusl, given the difficulty in adhering to these two vows?

If the vow was merely a thought, then nothing is incumbent upon you. However, if you uttered it, its ruling is that of a vow of obstinacy (Nathr al-Lajaj), where the accountable person is given a choice between fulfilling it or offering an expiation for an oath (kaffarat yamin). If you were under the influence of obsessive thoughts (waswasah), then nothing is incumbent upon you. As for the vow made to the mother, fulfilling it is not obligatory, though it is commendable.