I am glad you see that you are being a bit hard on yourself. I will say though, thank you for actually caring that you do a great job and take give great care, lots of new nurses do not really even know that they do not know something. Which is an even worse spot to be in-so you are doing something right!
Maybe it is the hit-or-miss nature of patient assignments is adding to your frustration. It is hard on a med-surg floor because you may get a hip tonight and a CHF tomorrow followed by an ETOH withdrawal mixed with a demented patient. All those difference can overload your brain when it is trying to learn.
I am not sure what kind of floor you work on however; maybe it would be possible to chat with the charge nurse and tell them-'Hey, I need to get really good at CHF with complications or I really need to learn more about insulins and timings(or whatever you feel)-can you throw these all of these types of patients my way for a while so I can gain proficiency with their care please?' So your brain can focus...
Maybe that gives you a chance to 'live' in the world of CHF for a few weeks straight so you can fully immerse yourself in the meds/labs/interventions/complications.
If that is not possible maybe pick a particular lab value to focus on for the month. BUN/Creatinine and the ratio, when it means AKF or CKF or when it is just dehydration. What would you expect the doc to do with each, to prove each. Look at the meds that would change the ratios, that may harm the kidneys. Sometimes coming at it from the back side-lab or med first could be a better way to cover all your patients. You know, compare the same lab across your patients. Why does one have a high BUN and the other no?
Just some suggestions. I think you will find that if you come at things from a slightly different angle it will get easier, faster. Focus on one thing and get good at it, instead of trying to be good at everything right now.
I am sending great thoughts your way-I know you will be awesome once you get through this first few months!