latin help! flowers are waiting on this...

Hi,

Can someone help me with some Latin? I'm looking to say:

"I am rock, build on me"

I barely remember, but think it would be something like:

"Petrus sum, in mihi aedifica"

It's a bit of a joke for my girfriend, playing on my name...

Thanks,

Peter

btw, since it's going on a note with flowers, how about "always yours" -semper tui?