The muscles inside the vagina (pelvic floor muscles) can be very tight. Also, if you're anxious about the fact that you are too tight, you will only get tighter and things will be harder. Remember that your pelvic floor is designed to be strong and also designed to be able to pop out a baby so it shouldn't be too small for a tampon. For the tampon insertion, relax as much as you can and stand with one foot on the loo or bath. When your period flow is heaviest this helps lubricate the tampon on the way in which can help. Perhaps you could start on a heavy flow day and get more comfortable using tampons from there. Try using a mini tampon with a silky cover and put some lubrication on it. It needs to go in about finger length so you can't feel it. If this still doesn't work, perhaps think about seeing a gynecologist for a thorough check up to make sure nothing else is going on. You certainly don't need to be having sex to be using tampons.