Weirdly, I have an inflatable one that I can’t use. I have bigger, so I have to assume it’s something to do with the material (my skin is evil, so this would not come as a surprise), but even only slightly in my butt begins, through the medium of serious discomfort, to yell “Nopenopenopenope”. It’s the strangest thing.
But I suspect this is not that; I suspect you need a smaller toy. Trust me when I say I know the appeal of going large, but you have to work your way up to these things. If I put in my bigger plug from standing it would ache like nobodies business and be far from pleasant. Get something disappointly small in the knowledge that it’s a journey.
Or get a anal training kit - a sliding scale of butt based plugs. Or even something like an aneros, which is like a small plug that neatly finds the good bits.