All praise be to Allah
Allah has said in the quran:
"Enjoin the believing men to lower their gaze and guard their modesty; that is chaster for them. Surely Allah is well aware of their actions." [24:30]
What you are doing is the zina of the eye. Zina means unlawful sexual intercourse. However here it means looking at something prohibited. As Allah's messenger(pbuh) has said:
Narrated Ibn 'Abbas:
I have not seen a thing resembling 'lamam' (minor sins) than what Abu Huraira 'narrated from the Prophet who said "Allah has written for Adam's son his share of adultery which he commits inevitably. The adultery of the eyes is the sight (to gaze at a forbidden thing), the adultery of the tongue is the talk, and the inner self wishes and desires and the private parts testify all this or deny it."Sahih Al-Bukhari Hadith 8.260
Therefore what you are doing is haram. You must restrain yourself from it as restraining from the forbidden is a form of jihad(struggle). Also you must pray because Allah has said to our messenger(pbuh)
"Recite that which has been revealed to thee of the Book and keep up prayer. Surely prayer keeps (one) away from indecency and evil; and certainly the remembrance of Allah is the greatest (force). And Allah knows what you do."
Therefore prayer keeps you away from commiting indecency and other types of evil. Also make dua to Allah to give you the strength to keep away from it. I advise you to fear Allah, remember that Allah is watching you, restrain from this for his sake and ask forgiveness from him.