The underlying basis is that most people think any new car company will fail. Period. For many people, no car company formed during their lifetime has succeeded. Tesla needs to not only make a car people want, they need to be able to deliver it in quantity and with quality and on time. That's a big burden - look at Fisker's woes. Add in that this company is also trying to create a new market for a type of vehicle that has been poo-pooed and has failed in the past, and which big oil and growing gas companies want to kill, and you can see why so many are pessimistic. That's all the long-term pessimism.
Short term, factor in that Q4 was the last of Roadster sales, so no new car sales until July (only deposits and powertrains), which means the company will report 2 quarters of even greater losses. That could lead people to believe that the stock will tank before Model S is delivered. Factor in that many people believe there has to be at least one schedule/quality/supply hiccup and you see why there's short-term weakness as well.
Since you believe in Tesla long term, my advice is to maintain that long term outlook. It's not unlikely that the stock will go down more before it goes up. Every time there's bad news from Fisker or on the Leaf or on the Volt, Tesla stock will be pressured downwards. Note that most people do not expect 200K sales of Gen 3 from Tesla, much less a P/E ratio of 10 for any car company. If you're worried about current weakness, then maybe you don't believe your own analysis or aren't patient enough to let it play out. You need to decide on your level of confidence and amount of risk you're willing to take.