The key is to stop trying to make it handle like the smaller iPhones. It will never handle that way. Can you do some things one handed? Yup. Will you get better at it? Yup. Will you ever master it like you did your iPhone 4, 5, or 6. Nope. It's just not happening, and that's ok. Can you one hand your iPad. Nope. You'd don't bother. It's not that kind of device. Can you one hand your iPad mini? Funny, because when I first got one, I figured, let me see what I can pull off. Laughable. It's not that kind of device. Neither is the Plus. It wants to be interacted with with two hands, and it's rewarding when you do. In contrast, it pays you with frustration when you try to make it act like your old, smaller iPhone.
I think of the smaller iPhones as phones that doubled as a media device. The Plus is a media device that doubles as a phone. Personally, I use the phone function of the 6S Plus about 5% of my total usage on the thing. A smart phone has always been a pocket computer and media device to me primarily. The Plus just makes all that experience so much better, and just happens to bring the best Smart Phone battery life around as a perk.