Ouch! I was told that the sachets just help to neutralize the sting, they don't get rid of the bacteria, so to always get antibiotics. I have had many a bout of cystitis and it is a nasty business - I hope you feel better soon
The sachets usually contain ingredients to make your urine less acidic -therefore less likely to cause pain/inflame on the way out. But if you have a full-blown infection you will need antibiotics to kill the baddies! Also recurrent bouts could be worth investigating further to make sure that everything is working as it should - immunity, kidneys, repro, etc. You have my sympathy - horrible illness.
Take paracetamol and neurofen for the pain and sounds like you need a course of ciprofloxacin (antibiotic). You have my sympathy. I had it a couple of weeks ago and spent the whole night gasping and wincing on the loo.