Pelvic floor muscles can be very strong. You are right, relaxing these muscles is the key to comfortable tampon insertion. Have you tried standing with one foot on a stool or the toilet? Also, you might like to try the tampon insertion on your heaviest flow day - this lubrication by the period flow helps. Knowing this can help you relax too. Since you have tried a few times and you are worried that something might be wrong, I think it’s definitely worthwhile to see your GP for a quick examination. There are some relatively rare conditions that block the vagina, which would mean that no matter how relaxed you are, it simply won't go in.