How to finish

My gf and I primarily have vaginal sex with a condom, but she has recently asked me to remove the condom and finish in her arse and she fingers her pussy -- we do doggy style and I have often fingered her arse (anus) with my thumb during the vaginal sex.

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The problem is that the pace of the anal sex is so slow and the fit is so tight and the entry is so difficult, I just can't seem to finish due to anxiety and have withdrawn and then squirted my cumshots on her back. Please offer advice on how I might finish inside-- my gf can't take the speed of thrusting in her arse as she can in pussy, and I fear hurting her.