Don't feel bad. If you are getting a good release of urine even part of the time at this stage, you are doing good! Pat yourself on the back! Seriously! You have to realize there are plenty of people who can't seem to get any good releases at first. You don't have to be perfect, fortunately the dogs do seem to survive our learning curve! It is possible she might get an infection eventually, but it is usually cleared up easily with antibiotics, so don't let yourself be too worried about that. I know you don't want your dog getting an infection, but it isn't the end of the world if it happens. Also, I wouldn't expect an infection this soon, maybe after a week or 10 days. Depending on your vet's judgment, sometimes the dog is put on a preventive antibiotic at first. I would think that would be more needed for the people who are having less success expressing and have to keep taking the dog back to the vet to be expressed. You could ask the vet about that if you are really worried, but it sounds like you're doing well for 3 days post surgery.
You might try taking her outdoors and laying her on a potty pad or something to keep her incision clean, and see if she'll do her number two. She may not want to potty in the house, but if you take her out she may find it easier to do her business. Another idea is, assuming you have normal sized fingers, you could probably try putting on a disposable glove and dipping a finger in Vaseline and just remove whatever waste is within reach. As long as you do it carefully she should be fine, the diameter of your finger is probably going to be about the same or smaller than the diameter of her normal stool. You might want to ask the vet for some lactulose syrup. It is a non habit forming stool softener, you only give a little and dogs seem to like the flavor OK. You can adjust the dose as needed.
Honestly, if you are already getting her decently expressed part of the time, you're already doing good and it is only going to get better. Just hang in there, this is all par for the course, and better days are coming.