You'll excuse me, of course, if I don't think think this is a good idea - I've already had one disappointment and I don't need another one.

I’m sorry, but I don’t think it’s a good idea. I’ve already had one huge disappointment, I don’t need another one.

you'll excuse me of course, but I don't think it's a good idea. I experienced one frustration, it is enough, no more repeat