I have difficultly breathing through my nose while trying to sleep, I take nightly antihistamines. Am I hurting myself? Will rhinoplasty help?

Not hurting yourself. By taking the antihistamine every night, but you're not doing yourself any good not being evaluated by an allergist/iImmunologist, you maybe allergic to dust mites or maybe just having chronic Rhinitis, until evaluated you can perform simple environmental maneuvers: keep pets away from bedroom, wash your pillows often, avoid smoking/smokers, check aaaai.org or acaai.org for an allergist in ur area.
No and yes. No, a nightly antihistamine is not detrimental to your health. And, yes a rhinoplasty with septoplasty may improve your night time breathing. See a facial plastic surgeon for evaluation of your nose and airway to assess your options.