As someone who was also abused in past relationships, this is my suggestion for you. Please understand, this is coming from someone who now LOVES anal.
The most important thing for you is that you are in control of the initial penetration. After that, well, it's more of a equality, but during the first penetration, it's all about you, and he needs to know that 100%. You impale yourself onto him, he does NOTHING you don't tell him to.
Now, if anyone else tells you anything besides what I am about to say, you tell them sweetlady says they're a fucking bullshit liar.... you need to use lube. Astroglide, hon, astroglide is the greatest gift of the gods to anal sex. Lube. Lots. more than lots. Tons. SPIT IS NOT ENOUGH. PUSSY LUBE IS NOT ENOUGH. EVER. Lube. Slippery, sleek, sexy, yummy, feels soooo good LUBE. When he thinks there's too much, he is just getting started!
Now, to start, you want him to just run it up and down your asscrack. Gently, lovingly. Like he's worshipping it. Reverently, tenderly. With a great deal of sensitivity and kindness. Then, he can begin to put a litle pressure on you... then slip it up and down along your crack again. Stimulate you, relax you, it's safe, it's sweet, it's loving. It's his gift to you, not yours to him. He is paying homage to you, lovingly. Keep this in your mind. You need this thought in your mind, for your sake. It's not an act of sex, it's not for him, it's for YOU. It's an act of him giving you pleasure. Believe me, dear, if you control it, if you keep the pain away, it will become a great pleasure for you.
Now, when he presses his dick against your opening, press against him, and he has lost all control, it's all yours now. That's your dick, you are going to use it for your pleasure. Let yourself press against it, thinking about using it for pleasure, let your ass start to part, then push out against him (yes, like you're trying to poop, but only with the opening, not from the inside, you'll know what I mean). As you push against him, that will actually allow his head to ease in further, when you feel the ridge inside (or if you feel a bit of pain), stop. Just stop and let your ass get used to the feeling... then slowly work it in and out, not so much that it pulls the lip out. Just a tiny motion. It will begin to feel erotic to you.
Ask for more lube, ask him to rub it around the tip of his dick that's inside you, then ease yourself back a bit futher. Now it's time to reach down between your legs, and rub your clit. Start feeling the excitement of both areas being stimulated. Move slowly, and each time you get a bit more of him in you, ease it in and out a tiny bit in a short, sweet rocking motion. HE MUST BE STILL, you are in control.
As you work him slowly into you, ask for more lube if you want it. When he gets buried into you, you will start to feel even greater pleasure. You will know when the time is right, when you start to move fast against him, you can release him from his being still, and let him have some control back.
If you're anything like me, at that point, you are going to want him to go hard and fast. And instead of fear, you'll be thinking how good he's loving you, how sweetly he's worshipping your ass, and how FUCKING GOOD! it feels...
(the penetration should be done on your sides. Once that's done, however, my favorite is for him to roll me over on top of him, and flick my clit quickly with his other hand [NOT the one that lubed your ass] and if you have one, a vibrator or dildo in your pussy at the same time will drive you absolutely wild. Try that sometime, and you may never go back

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