“Don’t worry, keep going…”
One day when I was in college I had to make a presentation with 2 guys (Dilip and A). Dilip was a good speaker and very charismatic; A was an extremely shy guy who never really spoke, but was brilliant. D and I hadn’t prepared anything beforehand and were just going to wing it as we normally did. A, on the other hand, had made a bunch of cue cards.
While A was speaking, at one point he suddenly stopped. It seemed that he had mixed up his cue cards. He froze. He started flipping through his cue cards and then he just stood there. I don’t know if maybe he forgot a cue card at home…? The teacher said: “Don’t worry, keep going”. Nothing. We told him to skip to the next section. Nothing. To this day, I’ve still never seen anything like it. He just stood there speechless.
What’s interesting about this story is that I’m sure A knew his material, but because he had prepared cue cards he felt he had to stick to them.
I’m not a fan of cue cards especially if you’re using a slide show; the bullet points on the slides should be your cues. I have a similar issue with preparing/rehearsing too much. You should definitely be prepared with general points you want to bring up but I don’t recommend having specific sentences or details fully planned out.
One exception: The peak of your nervousness is usually at the very beginning of the talk. You might want to prepare your first sentence or 2 so you can ease into the speech. Besides that, do your best not to memorize anything! 
“Grasp the subject, the words will follow.”
Cato the Elder
If you know your subject matter, what you say given the cues from your slides should be fine and it will look much more natural. If it’s natural, it’s much more convincing than reading cue cards or slides. A presentation that appears too rehearsed or too mechanical gets a little boring, is unconvincing and you wonder about the speaker’s credibility.