What is the ruling on my action, and is my prayer valid, given that I doubted whether madhy (pre-seminal fluid) had gotten on my clothes but was not certain, so I based my action on certainty, washed my private parts, performed ablution, and prayed?

If you doubt the emission of madhy (pre-seminal fluid), the default is that it has not been emitted, and clothes do not become impure merely by doubt.

However, if you are certain of its emission: 1. If you are certain that clothes have been contaminated by madhy but you do not know its exact location, you must wash the entire area you suspect, or the whole garment if the impurity is on it. 2. If you doubt whether madhy has touched the clothes or not, the default is that the clothes are pure, and you are not required to wash them, because doubt does not override certainty.

It is advised to avoid thoughts that incite lust and to occupy one's time with acts of obedience.