welcome to HCM,
as per the description u have given about the dermatological problem, it is most likely a fungal infection
known as TINEA CRURIS
... the lesions typically appear in red ring like fashion with central clearance and active outer border with red papules..
tea tree oil doesnt help you in this regard..
i advise you to use the following over the counter skin meds for your problem
1.TERBINEFINE CONTAINING CREAM - zimig or terbicip cream
CONTAINING CREAM - nizral cream
the above treatment will be sufficient for treating your problem , but to eradicate it completely and to prevent recurrence of the problem ,use those skin meds for upto 10 days extra period of time..becoz recurrence is more common in fungal infections