Making sure the area (and any other creases) are properly cleaned and dried well after bathing is important too. An antifungal sounds the initial way forward from comments above but once it's cleared up and isn't so sore and raw then sudocream is good.
Towel drying might feel rough on sore areas but you could get a softer soft or even make sure it's properly dry with some softer kitchen roll so the area inside the crease isn't left damp.
Also, if it's difficult to get to that area well when bathing then you could try using facial wipes (gentle antibacterial ones maybe or something soothing but unperfumed) to make sure it's all cleaned properly.
Try a really gentle wash, something like Sanex or Carex which is unperfumed and less likely to irritate your skin.
Talc is good but don't go overboard, go for a gentle one like baby powder and a blusher brush is quite a good way of applying it without layering it on too much.
Don't let the area get too warm or sweaty - a warm moist environment is not good. Tights can cause these kid of problems.
I found a problem having bigger boobs (underneath them), especially when hot in the summer. I also used to do a lot of care work volunteering and used to do a lot of bathing and making sure people with, for example, mobility (and a fair few weight) problems were properly washed to prevent problems.