I have only skim read the replies here so excuse me if I cover anything already mentioned but my first tip would be to not underestimate the power of a decent lubricant! I cannot take my larger girth toys without using a decent amount of good lube.
I doubt there will be a quick fix I am afraid. If you are big and she is small, well....imagine her trying to penetrate your bottom with a nice thick dildo, I am sure you would appreciate patience! :D
It seems as though you are already doing most of the things I would of suggested, like plenty of foreplay, ensuring she is relaxed (Has she has any problems before? Any medical issues?)
The only other things I would suggest is her practising alone with larger toys. I mean, vaginas are elastic and can stretch but afterwards, things ping back to her normal size, you know? So it may be a case that your new lady will need extended warm ups before each session, one finger, then two....you get the drift, until her muscle and skin elasticity has warmed up, stretched and prepped ready for you to enter more or less pain free. By the time you come round to your next session its starting the process again. Is she quite new to sex? I know I couldnt have taken some of the sizes I take nowadays back when I was new, although even now I have a comfort limit, anything about a 5 inch girth and I can't just bung it in and thrust like a demon. I need to warm up to it. Every single time. Because stretching is temporary. Relaxing enough to take a larger size, is something that can change I think. I am no doctor but I often suspect if I am pushing my limits, my muscles involuntarily tighten or clench. Its learning to stop that happening my going slow, so as to not force that reaction and relaxing more. Problem is, for me anyway, if I have tried to push past it when it already hurts a little, it just ends up hurting more and putting me out of action for a while from aching or even chafing or minor splitting...none of those are fun lol
So my tips I guess would be: Lube...plenty of lube seriously lube is a massive help. Warm her up as you have been doing. I think a bath is a good idea as it will relax her and get blood flowing and maybe add a little elasticity to the skin but don't expect miracles from a bath. Playing alone and practising with larger toys will help, if nothing else it will give her confidence about what she can manage, thereby reducing the risk of nervousness causing tension down there. let her keep going on top for as long as she needs. This will give you both confidence, she will relax more, knowing she is controlling the depth and angle and therefore less likely to hurt herself and it will give you an idea of just how fast and how deep she can manage so that you can mimic that when you try different positions. Speaking of positions, try lots. As many as you can think of. What works for one woman doesn't work for another and I know some positions do cause penises to feel deeper, but they may also open her hips out more, or the angle (Pushing into her back passage rather than her g-spot and bladder) might be enough to cause less intensity during penetration. You never know. Try them all and rule out ones that are a no-go for now and work on ones that work. Have patience. There can be many thrills to your sexy times that don't involve hard/fast/you in control penetration. :D I know it feels amazing etc but women sense things. If she senses your impatience in this, it may make it worse or she may feel like she is letting you down. :(
Hopefully things will get easier as she gets used to your size.