It's good that you know your body. The muscles inside the vagina (pelvic floor muscles) can be very tight. 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. It needs to go in about finger length so you can't feel it.