Is the distress, unease, and feeling of rejection towards the groom, as the wedding date approaches, a result of Salatul-Istikhara?

Istikhara (seeking guidance from Allah) is recommended for marriage and other matters. The preponderant opinion is that a person should proceed with their affair after performing Istikhara. If it is good, Allah will facilitate it; otherwise, He will divert it. It cannot be definitively stated that the anxiety you are feeling is a result of Istikhara; it might be due to fear of transitioning to a new life. You should supplicate frequently for Allah to facilitate what is good for you, and fortify yourself with adhkar (remembrances) and Quranic recitation for healing from every ailment.