Is it permissible to marry my cousin, who is ten or eleven years old, based on the Almighty's saying: "And those who have not yet menstruated"? And is it forbidden for my heart to be attached to her and to think about her as a motivation for studying?

It is permissible according to the majority of scholars to contract marriage with a minor girl who has not reached puberty, but it is not permissible to have intercourse with her until she is physically able to endure it, and this varies according to the circumstances of individual girls. As for becoming emotionally attached to her and thinking about her, it is advisable to distance oneself from such thoughts and occupy oneself with what is more beneficial, lest it lead to transgressing the limits of Allah.