Does the yellowish fluid found after istinja' (washing the private parts after urination or defecation) and istinja' from masturbation count as semen, and is what occurred considered masturbation with water, knowing that one suffers from obsessive-compulsive disorder?

Ablution with water (Istinja') is something prescribed by Allah and has no connection to masturbation. The fluid that exited after a feeling of lust is madhy (pre-seminal fluid), which is impure and requires Istinja', washing whatever it touched, and performing wudu' only. It does not necessitate ghusl (ritual bath) unless semen is discharged. These drops might be vaginal discharge, which is pure but invalidates wudu', or urine drops, which require washing the area and performing wudu'. Ghusl is not prescribed in this matter. It is advisable to persistently make supplication, stay away from obsessive thoughts, and consult a psychiatrist.