Is my purification valid, given my doubt about completely removing the impurity due to weak water pressure, and not repeating the purification to ward off obsessive doubts?

The cure for obsessive whispers (waswas) is to ignore them and pay them no mind. If doubt arises concerning the impurity of something, the original state is purity. This original state is to be maintained until certain and firm conviction to the contrary is achieved. What you did does not obligate you to anything, and you were not required to wash the spot whose impurity was doubted. The religion of Allah is ease, so do not make things difficult for yourself by giving in to obsessive whispers. Rather, strive against them and repel them.