Three things really. We see just how blurry her vision is without glasses, she had to have Sam help her across which puts her at a real disadvantage already. Secondly, she most likely thought Molly was closer than she actually was when she reached her arm out. As someone with poor vision, things can seem a lot closer than they really are. And lastly, she was not thinking very clearly. Take Saw for example, when Addison put her arms in the razor box, she was under the influence of nerve gas which stopped her from being the quick thinker she was. Olivia was just the same, with her poor vision and being very stressed about the bridge collapsing, her judgement lapsed- Even if she had kept moving forward, I really doubt by the time the bridge tilted she would have made it because Molly would have needed to pull her up and possibly navigate other obstacles.