Got a bit more girth than you mate so can understand this, my partner of 4 years still has problems sometimes.
Lots of foreplay, but I'm told don't use fingers because it will cause her muscles to contract to finger size.
Get her super wet, get her off first, then use lots of lube and go super slow, maintaining her stimulation. Try some very shallow thrusts, take a short break and then go again, if she can't take it all that time, don't pressure it, just go for what she is comfortable each time.
Depending on what your like also, do you find sometimes your not as hard/thick after you get off, maybe take advantage of that to get inside, while your still firm enough, but not bulging at the seems!
Time, dedication, and reassurance, if she is stressed/self conscience it's just going to make it much much worse. Failing that, maybe try some smaller sex toys to get her loosened up a bit first, then introduce yourself a bit between each go with the toy?