Go slow, use lots of lube, and stop when your partner says stop.

I suggest it is because you need to get your spincter engorged with blood to that it is soft and bulging. The way to do that (auto or with a partner) is to start with something smaller, like a finger and pump that in and out, and then move onto two fingers and then a group of three fingers. By that time the arsehole is wide and soft and supple and you should get the dildo in easy and it should squelch in and out easily.

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