Craig,

You are not alone in having this type of climaxing conundrum.

I believe its more mental than physical at this point. It's become a habit to come outside rather than inside of your lady. There must be a trigger that allows you to do this act and have difficulty with the other.

When you are having intercourse, are you in any particular role of dom or sub? I ask, because you've listed yourself as a switch.

You may need to go deep inside your mind to give yourself permission to ejaculate within her.

If she's in the top role, have her command you to do so.
If you are in the top role, tell her you are going to mark her as yours, or whatever turns you on and gets you past this point.

Are you using a condom?
That may help get past the barriers and lingering concerns of fathering a child.

With patience, practice and time, you'll be able to "train yourself" and implant new triggers to acheive your goal.

To your success,

Ruby