Kelly, have you told your parents what happened to you?
If you haven't, then you really need to do that. Being sexually molested was not your fault. You didn't cause it and you may need counseling in order to deal with the aftermath.
Counseling may also help you get over these fears you're having - an extra bonus, considering that you'd like to be able to experience life as it is meant to be lived (physical activities, sexual intimacy, children).
Meanwhile, if you want to try to get over your tampon issues without having to undergo therapy, might I suggest you do what most of us probably did?
Buy the smallest size of tampon you can find, then lock yourself in the bathroom and read the directions about a hundred times.
Take a tampon out of the box, open it, read the directions again and then try to insert it. Wind up throwing it away because you did it wrong. Repeat.
I know that sounds silly, but what I'm getting at is that you have some extra fear because of what happened to you - but the rest of it is perfectly normal and usual for a first-time tampon user.
Not all brands are the same and not all feel the same. Experiment until you find a brand and size you like, then try to relax and go from there.
Meanwhile, here is a link with information which may help you with the effects of the sexual abuse you suffered. I hope it provides you with some answers.
www.sandf.org
Take care,
jkgrandma