The fact that you have been experiencing recurring episodes of cystitis
could raise doubt on some underlying condition that could be causing the same.
Cervical erosion is a diagnosis that could br assumed much later, although your age and history does support this, I shall leave the commenting to your doctor after an elaborate evaluation.
For now, I advise an examination of your urine anda vaginal swab test to find the exact cause for your sympoms. Also, get a culture and sensitivity examination of your urine or vaginal swab, in order to ensure rapid and effective treatment, and avoid future recurrences.
In the meantime, please avoid all forms of sexual contact.
Hope this helps.