First of all, why on earth are you shouldering all the guilt and blame?
Your cousin made the overtures, he was the one who opened the door.
Go easy on yourself.
However the feelings you are currently going thro are a very good reason why interacting with the opposite gender is best kept to the minimum and business like. Also next time, make sure your family is fully involved so you dont get to a stage of being deeply emotionally bonded with the person before your parents approval.
Pray salat, ask Allah to help you let go, and forgive yourself.
Your cousin will get over it, time is a great healer, in the mean time it is prolly best for your healing that your cousin stay away from you all for a while.
In the meantime I'm sure we will all rmeember you in our duas inshallah, and I hope Allah rewards your patience with a worthy spouse, one who makes you and your family all very happy too inshallah.