Having received a negative outcome of a marriage related Istikhara performed by my aunt on my behalf one year ago, I am clear in my head that I want to be with him. Should I perform the Istikhara again by myself?
Bismihi ta'āla
Please refer to previous answers regarding this topic of istikharah and marriage. They will be useful for you to read. Should you and your family have done due dilligence in investigating him and how religious he is and his high akhlaq and there is compatibility between the both of you, rely on your judgment and corect evaluation, rather than istikharah.
And Allah knows best