What kind of a little dog is she?
The main thing I can think of is just that expressing after surgery isn't the same as expressing at normal times. When my dog was spayed and I had to express her, for one thing it seemed like she had a lot of gas in her abdomen which made the bladder hard to get hold of. I had the advantage that I'd already been expressing her for a year before the surgery so by then I was good at it, but it was still hard to do for the first 4 or 5 days after her surgery. I'd say all you can do right now is do the best you can. I think that if my dog had had pain medication following spay surgery, her abdomen might not have been as rigid. I don't think she had any pain meds, after all she was paralyzed, you would not expect her to feel anything, but I look back and suspect she did feel discomfort from the spay. (She can feel her abdomen very well now.)
When my dog's abdomen was rigid it helped to do deep tissue massage were I kneaded her abdomen till it relaxed. Of course that is the upper abdomen, I could not massage where her surgery was, but it helped a lot. Then her muscles relaxed and I could express her better. I just don't think you can expect to do a perfect job right now, don't blame yourself. Like I said, I was already good at expressing and I had a very hard time right after the spay. One other thing that may help is if you be sure to keep her warm enough. It sounds like a small thing but my dog was cold after her surgery, and you do tense up when you are cold.
You might talk to your vet about keeping her on preventive antibiotics for a while longer since it isn't easy to express right now. It's going to get better, you are just having a hard week as anyone in your situation would.
If it turns out she needs to wear diapers between expresses, you can always do that, and if she is as small as you say, you will have an easy time getting her diapers at the grocery store, you just cut a hole for the tail. However, she may not need them after her surgery heals and you see how you do with the expressing then. Did you see the information on diapers?http://handicappedpets.biz/wiki/index.p ... male_wraps