It is very difficult to give advise only reading one perceptive on a situation. I have worked in multiple hospitals which has given me the opportunity to work with many preceptors as well as having acted as a preceptor. One question I have regarding your situation would be, where was your preceptor for the 4 hours that the levo was not titrated? Your preceptor should be keeping a close eye on you as you are still learning. Your preceptor should have asked you what your plan was regarding that patient if she had noticed you did not titrate it. I personally would have used that aa a learning opportunity to see where you where at with critical thinking skills, knowledge of medicine and rational for your actions. If as a preceptor I noticed a bp of 80/40 and did not see you take action I would question you, if the pt bp was truly 80/40 I would be in that room overseeing what your next actions where, what other vitals are you looking at, why did the bp suddenly tank or was it trending down. A good preceptor should be listening as you presented the situation to the Dr. which also gives them an opportunity to see if you understand the whole picture.
I had a new RN that was "drowning" for lack of a better term and I stopped her and asked her what was going on, where was she at with med's etc and I helped her out. I stopped her because one I was supposed to be training/teaching her and two because patient safety is first and foremost. After our shift I explained the importance of asking for help and how the other patient could have been adversely effected by her being caught in the other room. I explained and showed her examples how on a unit particularly an ICU we need to help each other.
My advise is don't give up if you love the ICU, talk to your manager for true and constructive feed back regarding your training. Get specific examples and evaluate how you could have changed the situation. Take what they say no matter how harsh it may be and learn from it. Find another ICU and ask to shadow see if the preceptors want to teach or if they are forced into training new RN's because of years of experience. Good luck in your future.