You did not provide photos so it is difficult to know with certainty, but you may be one of those rare patients who begin to develop a little fibrous tissue in the lips with years of treatment (which happens especially if a non-HA filler has been used, by the way). This can be resolved with prescription medications Protopic and Retin-A. You might need just a drop of an HA filler as well, and #Beletero or #Restylane would be good choices.
If you want to see someone in LA who is very familiar with this problem, I can recommend #Ava Shamban, MD.
Without photos this is hard to answer specifically, so either post photos so we can answer better, or choose some respected physician injectors for some in-person consultations. In general, your choices would be to add a bit to the other side, or to have the tiny bit extra on the right side removed. But it would be best to get on this asap so you have time for swelling to resolve before your event in a month.
"This answer has been solicited without seeing this patient and
cannot be held as true medical advice, but only opinion. Seek in-person
treatment with a trained medical professional for appropriate care."
Hi Lil10. It's difficult to answer your question appropriately without seeing pictures of the issue you are having. The best thing to do is either post a picture, or be seen for an in-person evaluation. Juvederm in the lips can last anywhere from 9 months to 1.5 years in some cases. If you do need additional product then I recommend getting it done now before your event. That will give enough time for the swelling to subside.
To see before and after pictures of lip enhancements that I have done for patients in Los Angeles using Juvederm and Belotero you can click on the link below. Hope this helps.