What is the ruling regarding my fasting and prayers after I used to engage in sexual fantasies during the daytime in past Ramadans, with traces on my underwear that I do not know were madhy (pre-seminal fluid) or manyy (seminal fluid)?

Sexual fantasies do not invalidate the fast. The emission of mani (semen) solely through thought does not invalidate the fast according to the majority of scholars. If you doubt whether what was emitted is mani or madhi (pre-seminal fluid), you may choose and assign it the ruling of one of them. If you prayed after something was emitted without purifying yourself, your prayer is invalid and must be repeated. However, if you performed ablution (wudu') after these secretions were emitted and washed them off, your prayer is valid. If you are required to make up prayers and do not know their exact number, then make up what will achieve certainty or a strong presumption of discharging your obligation. If you doubt the time of puberty, then add it to the latest time it is likely to have occurred.