Lubrication is everything ! The rule of thumb is that too much is almost enough.
I use a total of four lubricants and with some rare exceptions, Its great for me and the guys love it.
First go buy a device called a lube shooter. or a syringe so you can inject lubricant above the anal sphincters. By having lube above the sphincters, your cock actually becomes more lubricated as you penetrate him. I personally use a 20cc syringe.
Start by having him insert 3 KY liquibeads, These are little eggs of Lubricants which will dissolve over 20 or 30 minutes or so, giving you long term lubrication.
Secondly, I insert two lubes internally. One which is ID platinum which is a liquid silicone. The second one is called Albolene and can be bought in any pharmacy. The albolene melts at body temperature but has a different feel that the silicone liquid. The mixture of the two is awesome.
Lastly I coat his dick with Astroglide Diamond Lube. Which lasts well.
You sphincter will ultimately relax and can accept any size cock. Any pain after the initial insertion is probably lubricant problems.
One last thought. When you finally get all the way inside him, who starts pumping, him or you? If the answer is you, that may be the problem too. As a smaller size cross-dresser, 5'4" all cocks seem big. It is painful to get a guy inside me, especially if I haven't had sex in a while. Once he is inside me, I need a few minutes to get used to him being inside of my body. If he immediately starts thrusting, its painful and miserable for me, though I would never let on until I got older and wiser.
Now when I take a man inside me, I ask him to be very still for a few minutes. Sometimes we kiss or cuddle, but mostly I like to just "be there" for a few minutes as my body adjust to being taken by a man. After a while the pain goes away, I can start pushing onto him a bit. After a few minutes of this, I Start pumping slowly and then a few minutes later, I am totally good to go. He can do whatever he wants after that.