That is funny that your nurses said NOT to shower with the pouch on, because both of my ostomy nurses said the opposite--they said to feel free to shower with the pouch on because it is made to withstand the water...!
I am also a 3-week old ostomate and I have showered daily with my bag and had no problems with the wafer coming lose at all....I think you should try a new type of wafer! I use the wafer that goes with the Coloplast Assura line...I think I lucked out because it seems to work really well for me! They make specal extended-wear wafers also--maybe one of these would be best for you?
I just want to get some of those bags that are shaped specially for your body (I am not sure how to describe them)--my current bags are just the normal oval shape, but the contoured ones seem like they would fit better under my clothes!
Anyway, call around and get some samples of other systems... I have already gotten a bunch of free samples of things like skin prep, "intimacy bags" (REALLY small ones for those times), etc.