You need to be seen today or tomorrow to have this looked at and swabbed for herpes. You yourself can't decide if it's herpes or impetigo - it's why we go to the doctor to discern this!
Could your current partner of 8 years have herpes and not know it! Definitely. You and your current partner both would need to be tested for herpes with an IgG type specific blood test. Assume nothing. But be seen asap and be sure the swab it for you.
herpes or intertrigo. How can I tell the difference?
intertrigo probably wouldn’t burn and would be in a chafed area.
If one of us was infected by a previous partner could it just now be manifesting?...
Yes, people can go years without signs/symptoms.
Does this mean maybe it isn't herpes??
It could be a lot of things. Many harmless things besides herpes. Go to the Dr. You may just need antibiotics.
Using the internet to try to diagnose yourself will just add to your concerns the only way you will know is to see a medical professional and take a test. There are free clinics or planned parenthood.