I’m glad to hear that you’re not afraid to use plenty of lube. Make sure it’s also a good quality lube though, J-Lube and Astroglide are both really good, though J-Lube is sugary and can cause yeast infections in women, so proceed with caution. 

If you haven’t tried them yet, poppers really make a difference too. I wish we’d discovered them sooner, because they relax your muscles when inhaled, and would have probably helped us manage fisting earlier (it took us about a year of dedicated practice without them, and then only a few months after we did try them I got his second fist in me). They are medically safe, and available at local sex/head shops, and online. We usually get ours from PictureBrite.

(That’s a popper bottle in the lower left)

For specifically opening up that ring of muscles, I’d suggest a long inflatable. The ones without a core are ok. It’s a little uncomfortable, but you can fold them in half, insert them, and then inflate them, which might make them the best choice for a beginner. Be careful not to fold it when you store it though, or it will eventually become weakened along the fold and split (not that we were dumb enough to do that of course…). 

Personally, my top choice would be a long inflatable, *with* a solid core. It’s more a bit more fun and comfortable to insert right at the beginning, and I suspect that they’re just a bit sturdier in general. This is my current model, and it’s lasted me quite a while.

Good luck!