I was sexually harassed when I was young but I was still a virgin.Then I lost my virginity to a boy. After few months we recited nikah ourselves, as our parents were not agreeing. He is a sunni and I am a shia. There were no witnesses when we recited nikah. Is my nikah valid?

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As you mentioned that you were virgin, therefore permission of father was a must at the time of Nikah. And whatever sin you did as you mentioned above, please do isteghfar for that. You are saying you have recited Nikah yourself, I dont know what you have recited. There are specific sentences which are supposed to be recited correctly and the one who recites must understand therefore it is hard to say that it was correct.
Please do isteghfar, get permission from your father, get blessings and prayers from him and contact local scholar for Nikah. Boy is Sunni therefore as per the ruling, you must get assurance that you will be allowed to practice your faith.