The advice above re positioning is on-point. I had a fuckbud for a while who had a little over 10 thick inches, and the key to full penetration was to put me on my back with my butt on the edge of the couch. He would kneel on the floor and he'd press my knees into my shoulders and put my feet over his shoulders or he'd hold my legs open. Being reclined like that with my butt angled up not only line my hole up perfectly with his cock, but it also put me in a natural and easy position for him to take his dick to the root. He was very good at opening my deeper sphincter; he'd work it for a while just slow thrusting until he felt the resistance, and his cock opening me up and pressing against it a bunch of times allowed for it to eventually give way. I'd always be delirious with pleasure and unable to form words or thoughts but he'd always comment along the lines of "there we go, now your second hole is relaxed and opened up for me" and he'd start deep dicking me with every inch and not finish until he shot (usually) 3 loads in me.
I always figured eventually I'd gain more control over it, but never did - it's not as easy as controlling the main sphincter. But thankfully he was patient and really enjoyed the process of opening me up every time we'd fuck. Once it opened it would stay relaxed, so the marathon hole fucking he'd give me would always involve his full length for the duration.
Practice with that positioning and you should be on your way to indescribable pleasure real quick.