hi mizz manna xo...
you have anxiety issues and anxiety issues (like panic attacks) are famous for making phobias.
being as it is, you have to continue to fight the inclination for your brain to create a new phobia constantly (my guess).
there are a couple of things you have going for you. 1) you have taken showers ALL YOUR LIFE without consequence. 2) you know and understand that this is an irrational fear.
here's what i would do, and i'm no-body, i don't have any special knowledge, etc. just my 2 cents.
go in the bathroom at your house. get naked. turn the shower on. put an arm (up to your elbow) in the shower. consider it a victory and stop.
next day, put an arm and a leg up to your knee.
next day, wash your arm up to your elbow and your knee down to your foot.
keep doing this type of thing until you get a little more and a little more of your body in the shower.
don't expect it to be easy. it won't be. it will be hard and you are likely to panic like HELL, but remember all the time, that this (showering) will not hurt you. it has never hurt you and that's that.
i had a roomate who had severe panic issues and this is the approach she took when she had a phobia of leaving the house. this is what she did to just get to the parking lot to the mail box. it took her a few days, but was well worth it. now she can go out and go to the grocery store, go to work, etc. she still has big problems (so it doesn't work perfectly) but guess what?! she can go out now and she couldn't before.
if you'd like for me to be your pal and encourage you, i'll be glad to.
good luck!
jasmine