My girlfriend used to see our anal sex as a one-way treat for me. She took a lot of warming up before she let me start thrusting or going deep. We were doing a good dose of foreplay, lubing well, and using a buttplug as a warm-up, in place while we did oral and vaginal sex.
The plug we used I assume is one of the most popular types ... stainless steel, teardrop shaped, and with a jewelled base. Even the small 1.2 inch plug was slow going as we eased it in millimeter by millimeter into her, slowly opening her arsehole until getting to the widest part, after which it would suddenly pop inside with her anus closing rapidly and clamping down onto the narrow shaft between the knob and the base. We would then do all other forms of love-making before returning to her butthole. I would slowly pull the buttplug, watching her anus slowly expand to let the thick part emerge and finally pop out.
The thing is that even with the buttplug in place for half an hour or more, her asshole was only acclimatized to the narrow neck of the plug, way thinner than even a single finger.
Once I realised this, I bought a type of plug more like a cock ... longer and more tapered. At its thickest part, it was 1.5 inches, a bit narrower than my cock, but importantly it only narrowed down to about 1.2 inches at the part that her sphincter clamps onto near the base. Her rectum also had more of a cock-filled sensation.
After normal love-making, withdrawing this plug was smooth and easy, and left her gaping for a few seconds, ahead of my lubed cock squeezing its way into her tightest hole. With her ass acclimatized we were able to commence deep steady thrusting right away. She doesn't tense up any more, and is comfortable before and during our anal, and loves buttfucking now about as much as I do.
So, try changing from jewel-based to cock-like buttplugs ... it has completely transformed our buttfucking!