Why is my nostril sore inside all the time?

Vestibulitis. We generally refer to this as vestibulitis. Most common cause is trauma to the tissues (picking your nose, wiping your nose with an abrasive paper towel, etc). If you get this, gently apply an antibacterial ointment (like bacitracin) to your nostrils several times a day and avoid trauma! if it does not get better in a few days, see your doctor to rule out other possible, serious problems!
Good Question. Do you ever put anything up your nose (not being funny) but otc or prescription nose sprays? Have you tried putting a topical petroleum jelly in your nose to soften and lubricate? Do you get bloody noses? Maybe you should see your allergist or ENT to have them examine your nose thoroughly.