I am happily married for 15 years and my wife and I have great sex. My penis is longer than average but not enormous. Occasionally during sex, I bottom out (for lack of a better term). While we are going slow this seems to give her great pleasure, but if we are going fast this causes some degree of pain. Can anyone out there give me some perspective from a woman's point of view? Any ideas on trying to prevent this from happening (besides the obvious don't go in as far which is sometimes hard to do in the passions of the moment)?

OK...

As a guy who's been through that. It's perfectly normal. But just be a respectful, loving partner.

You have to "trim up the main sail" in certain positions... And don't harpoon her.
That flower is fragile. When you are giving it the steep and deep in the "passion of the moment" as you say - have her tell you what position she prefers... Missionary with legs at safe angle can be good... behind can hurt if you forget what your parameters are... just go slow and see where you are at in regards to her anatomy. I'm sure you will get some women her who can talk to you about communicating on this subject and learning her body. Learn hers and yours... that's what being a good lover is all about. No reason you should hurt her.

That has less to do with you length, unless you are like 12" + than it has to do with her personal anatomy. Some women can take more than others. Trick is if this happens frequently is find another position where it has her in control or has less penetration. After all causing her pain is the last thing you want. And there are a LOT of different positions.