Sorry that you're going through this! It is truly uncomfortable and painful to suffer with.
I suffered with this too for a few years, and every month was the same - so uncomfortable to wear underwear or sit down. I can completely empathise with you as it is so excruciating. One month I decided to completely clean up my diet, and the soreness went away.
I cut down on processed foods and drinks such as processed meals, desserts, white bread etc. and started eating more raw vegetables and healthy fats such as humous, olives and nuts, and I also significantly increased my water intake, by drinking more water more often.
I don't know your current dietary habits, but I had to make a conscious effort to eat better throughout the month. I don't eat super clean all the time, nor have I completely cut out the foods mentioned before, but it's just about making better choices and eating less of these things.
Throughout our cycles and especially as we get closer to our period, our vaginal pH levels increase. Menstrual blood can also increase vaginal pH levels. Therefore, more acid-forming foods such as sugar, bread, dairy, etc. cause more acidity in the body, which causes our pH levels to increase. And, when we are out of balance for too long, bacteria has a chance to thrive and cause discomfort due to inflammation.
As your normal monthly cycle can cause hormone changes which in turn alter vaginal pH, perhaps your swelling and soreness decreased because the pill generally maintains more consistent hormone levels.
I really hope that this has helped you in some way as this has worked for me, and I really hope that this problem goes away for you as I know how it feels. Try and give these things a go and see how you get on, I really hope that you feel better soon!