@muri,
Sex is more "mental" than anything else. The more you are turned on, the quicker you will orgasm, and that is why it is quicker when you do it yourself.
Maybe it is that this particular girl really doesn't turn you on that much. This is the kind of thing that happens sometimes when a couple has been married for a long time. Things get stale and need re-energizing.
Ask yourself, what could this girl do that would turn me on more? Maybe she's overweight and that turns you off. Or, maybe there is something else about her that you try and overlook but in reality is a turn off.
Think!
.