Yep, me too, as I've been fond of quoting, "I like sick people, but broken people will work too."
The complicated patients were always the ones that made me step-up and really stretch my abilities as a student nurse. From what you said, you did more than you may know. Due to the pace of nursing, your patient might not have had much interaction in the last couple of days. Just by being there, providing at least a modicum of care, understanding and compassion may have been the best thing for her. Sure you didn't get to start an IV, push meds, dress a wound or any of the myriad technical skills that as students you long for, but you cared for her. It may be hard to see it, but you did something. And yes, you will get many more of those patients where you know next to nothing about, either from lack of coverage in class, or from lack of experience in the clinical setting, but you can always go home and look it up, or take a moment on the floor to look it up, it's how we learn. You can 't or shouldn't be expected to know everything, it's not possible. Go with what you know, which it sounds like you did, the rest will come.