Is a girl considered sinful against her husband, from whom she was asked to seek Khulʿ, knowing that she left him after he had supported her financially and emotionally, and she now feels no love for him and has abandoned prayer?

There is no legitimate marital relationship between you. What you did is not a legitimate marriage contract, as it requires the presence of the guardian or his representative and the testimony of witnesses. Merely reciting Al-Fatiha does not make it valid. However, since you both believed it to be permissible, its dissolution requires either a divorce or a ruler's decree. The waiting period (iddah) is obligatory, and lineage is established if there is a child. You both must offer sincere repentance and cease this illegitimate relationship. Furthermore, abandoning prayer is one of the gravest sins, and you must hasten to repent and maintain it, for it is the pillar of religion.