What should I do concerning this young man whom I loved and became attached to, and whose mother refuses his marriage to me, knowing that I cannot live with any man other than him, and that our communication is minimal and free of illicit acts? Is this communication of ours unlawful? And what is the reason for this pain I feel despite my religious commitment?

You must turn to Allah and supplicate to Him to achieve a righteous marriage, for relationships outside the framework of a proper marriage cause much evil and tribulation, and the Sharia has warned against fitnah (strife/temptation) between men and women. Complete repentance from such relationships requires cutting off all communication entirely. Marriage is the best path for two people who love each other. If marriage to this young man is possible, then it is a good thing. Otherwise, patience and contentment with Allah's decree are necessary, along with entrusting the matter to Allah, for He knows best the consequences of affairs. The cure for obsessive love is easy, and the statement about not being able to live with any other man might just be an illusion.