Concerns about 100mm Macro USM - can it be sharp without IS or expensive equipment?

I have a Rebel XSi (crop sensor). I just purchased a 100mm Macro USM (non-IS). I prefer handheld shots but I just can't afford the extra $400 for the IS version. Now I haven't received it yet...but I have concerns that my images will be blurry because it doesn't have IS, I don't have a flash, and I don't have a great tripod.

I want to take macro pics of bugs, flowers, nature stuff. So I understand I will need either great lighting or something to steady the lens. I have a cheapo tripod and also a monopod, but I think that's still pretty restrictive for chasing around bugs.

Did I make a mistake in buying this lens? Will I still be able to get good photos with it handheld? Does anyone have any tips? I can't afford an expensive tripod or flash. Would a cheap flash or LED ring light help at all?

I was also considering some alternatives like extension tubes or a Raynox DCR-250/150. How would this dedicated macro lens compare to say, a 50mm f/1.8 with extension tubes, or 70-300 IS USM f/4-5.6 with a Raynox DCR-250/150? Could an IS lens with a macro attachment actually be better under my circumstances, just because of the IS?

I've always wanted a macro lens, I just have these doubts because the rest of my gear isn't great. If anyone has any tips on how I can get the best quality I can out of it, please help.