In that case, for starters I’d recommend positions where you don’t have to flex any muscles to hold yourself up. The mental part of relaxing is definitely the biggest hurdle, so it helps to eliminate everything else so that’s the only thing you have to spend effort on. Then you work to eliminate all of the mental blocks and stresses.

As part of the preparations, put on some music or white noise, focus on breathing for a bit, any exercises that help you get into an almost meditative state. Then, tease yourself, don’t even insert anything right away but touch yourself on the outer rim of your opening. Go in circles, make your anus comfortable with the non penetrative contact. You’ll eventually get to a point where your body is PHYSICALLY craving more, but that is very unlikely to happen if you rush it.

When your body begs you to put something inside of it, start with something smaller first (well-manicured fingers, smaller “training” toys) and then work your way up once you feel like you can comfortably go up in size. Make sure it’s what your body wants and not what your brain thinks would be hot to make your body do. Stretch yourself a bit, little by little with the smaller objects; this is where fingers do a marvelous job if you’re ok putting them down there.

It usually takes a fair bit of time the first couple of sessions to get your body comfortable with the sensations from anal play.

I hope this helps! My first few times were pretty rough and advice like this is what helped me make the experience more enjoyable.