Congrats on your new instruments!
I can picture that it is in fact overwhelming. While it's a lot of positive emotions, sometimes it can be exhausting for a while.
SX900 is the newest top-of-the-line PSR instrument, and a lot of people find that this sort of instruments is rather complicated. Surely, it's not hard to sit down and play, but it's hard to know that there's a way to use some functions and not to know how. So owning PSR is a sort of commitment.
Owning P121 is a different kind of commitment, it is more of a physical challenge. I have PSR with very light keys and Juno DS with a bit heavier keys, and often I feel that I'm actually lazy to deal with extra resistance. In the end, playing keyboards is not watching a TV-show or eating icecream.
Even when such attractive products like SX900 are captivating, we can not expect to be entertained by it endlessly, especially when the first excitement is over. I don't believe in forcing oneself to play, but I think that staying inspired and motivated has something to do with, well, everything, with what we are dealing with within our minds, with things we are experiencing socially, with music we hear.
I could've said that sometimes you just have to sit down and play, but I just know that there're things that I do, like sitting down and play, and thing which I don't do, like playing with a ball, and this is what I consider to be a big challenge for manufacturers and sellers - no matter how kool is the new PSR, you can't force to play all the people.
But you are surely don't need forcing, I bet, Yamaha is currently disrupting your routine in a major way :-)