How do I know if I have nasal bridge collapse & how would it affect appearance of my nostrils & tip?

Would recommend you talk to your septorhinoplasty surgeon to determine if the nose has fallen.Much of that depends on how your nose looked before the trauma, and before the surgery.Surgery and trauma can both affect the height of the nose.

It is difficult to assess what is going on with your nose based on your descriptions alone, without any pictures. However, based on what you've written, it is possible that you do have nasal collapse.  The fact that you had nasal trauma, nasal surgery and current difficulty with nasal breathing suggest that you may have anatomical blockage some where.  The only way to diagnose anything accurately is to be examined thoroughly by a facial plastic surgeon.  Also, many anatomical changes can happen as the collapse occurs--- again, you need to be examined to be evaluated for that.

 A  rhinoplasty procedure can accomplish repairing the bridge collapse after trauma. Osteotomies placed in nasal bones, and structural cartilage grafting can accomplish straightening and building up the bridge line. A septoplasty procedure may be  helpful to improve the airflow through the nose, which must be documented at the time of examination. For many examples  and more information, please see  the link and the video below