I could have sworn I read somewhere that you already bottom for your partner. You _should_ already be experiencing the P spot orgasms. Maybe I shouldn't assume this. Maybe you are talking about another type of P spot orgasm, could you elaborate what you mean? Are you talking one of the hands off (not touching yourself) orgasms?
In anycase I was at a Adult book store shop and I saw that Chi Chi La Rue came out with some sort of curved or ribbed dildo ... Looks like it would do the job. What I liked about it was that it didn't seem as flimsy as the jelly ones. So its semi firm, but not rigid enough to create a new hole should you become too excited with it and miss the spot.
Let me see if I can find a link to it:
Try different angles I guess. If you aren't really experiencing these types of orgasms, I've come to the conclusion (and I'm no expert) that you probably don't ejaculate much to start with or you might be hitting a psychological barrier (ie strict tops). Myself I cum a lot and my p Spot gets swollen pretty easily, so anyone with a 4inch penis can do the job.
The hands off experience is a little more difficult, but a stiff partner with a decent amount of girth can get u to do this with enuff time. I also find its psychological... like i gotta be excited as heck... but in that case the wind probably would make me cum if it went up my ass. Sometimes it can be painful. In general its kinda hard to fight the urge to stroke yourself though.
Thats my two cents ... good luck