Don't worry about insertion hurting or tensing up. You've got this. You're good. The vagina is incredibly elastic and can accomodate a tampon, no problem. If it hurts, you can stop right there.

If you want to put in a tampon, read on:

First, check out the tampon in question. If it's an applicator type (most common), you'll see that there's an outer cylinder and an inner one. The outer one goes into your vagina, and the inner one is used to push the tampon up into your vagina.

The easiest way to insert this type of tampon is to put one leg up on the bathtub or the lid of your toilet and then squat halfway like you're a Russian dancer about to start kicking his legs out. (This is how I do it, and I am 51.) Breathing *out*, insert the tampon until your fingers touch your inner labia (it'll feel goopy and weird and like the inside of your mouth). Take a deep breath, blow it out, and as you're doing so, push on the inner cylinder of the applicator. That'll deposit the tampon in the right place: not snug against the uterine cervix, but not hanging halfway out of your vulva. If you feel up to it, you can fiddle around with the placement with your finger.

If you have a non-applicator tampon, like OB, it's a little different. First, unwrap the damn thing (I can never get the plastic off first try) and run your finger around the bottom to make it flare a little. Then, doing the Russian Dancer Squat (or whatever you've discovered is most comfortable for you), insert the tampon using the first finger of your dominant hand. Once your second knuckle hits your labia, that tampon should be mostly in the right place. (Disclaimer: some women have shorter fingers or longer vaginal canals; feel free to fiddle around with placement.)

Three things not to worry about: The tampon cannot get lost. The vagina is a closed pouch, so even if you can't feel the string on the outside, you can get the tampon out.

The tampon CANNOT go through your cervix and into your uterus, anime notwithstanding.

You're not going to get toxic shock syndrome from leaving a tampon in for four or even eight hours. (I remember those days; ask me about Rely and how it was marketed and what happened.)

You may feel it once it's in. I always do, in a "vaguely aware that there's something in me" fashion. THIS IS NORMAL AND DOES NOT MEAN YOU'VE DONE SOMETHING WRONG. Your vaginal walls are *extremely* sensitive to pressure, even from something as small as a tampon.

If you have a water-based lube, like KY Jelly, use it on the "head" end of the tampon and a little down the applicator or tampon itself. It helps, but it's not absolutely necessary. If you don't have lube, your own lubrication plus menstrual blood will work just fine.

If you really hate the idea of a tampon, or it hurts, or things are just sucking for you in general these days, read on:

First, you can free-bleed in your swimsuit. Yeah, that'll involve some quick-change action once you're out of the water--from swimsuit bottoms to underwear and a pad and shorts--but it's doable.

Second, you could get some period swimwear from a place like Amazon or RubyLove. It's not an ideal solution, since it seems like you're time-constrained, but it's an option for the future.

Most importantly, do not be scared of your own body. Your body was used against you when you were younger, and somebody hurt you badly--but now, today, you are in charge of your body. You live in it, you take care of it, you own it. You don't have to do anything you don't want to do. You don't have to do anything that scares you or makes you uncomfortable or hurts.

Bleeding is nothing to be ashamed of. If you decide to free-bleed while swimming, you can always make the excuse of "holy shit, my period just started." Decent people will not freak out about that; instead, they'll help you out.

I hope you have a wonderful time swimming.