Is it obligatory to fast 30 days after repeating Wudu (ablution) forgetfully, knowing that an oath was sworn to fast this period in case Wudu was repeated, as a means of self-discipline, and is an expiation for breaking an oath (kaffarat yamin) required?

Whoever breaks an oath out of forgetfulness is not bound by anything, according to the more preponderant opinion. The oath of a person afflicted with waswas (obsessive whispers) is not binding at all, as such a person is considered to be under duress. It is advised to strive against waswas by ignoring it, and not to use oath-taking as a means of struggling against it.