The end result of an existential crisis is that you end up staring out into a 'nothingness'. The are no great answers to why you are here, but you are here. There is no great purpose to pursue, no ultimate goal, no grand plan. There is no afterlife that you know of because an after life is just that... after life.
So what you have here is life, this life, with no reason why you were thrown into it or why it's so short or how come you got stuck with the body you've got or the parents you got or anything else. The only thing you know for sure is that you are here. The only question you need to answer is what are you going to do with this 'you are here'.
After you've stared at this nothingness for a while, you realise two things. One, this existential questioning is going to emerge for everything you do. Two, you have complete freedom to choose to do what you want. You want to be a doctor, be a doctor. You want to study chaos magick, study chaos magick. You want to travel the world, travel the world. You are responsible for your own life. Do what you want with it.
The existential crisis, after the realisation that you are 'condemned to freedom', then engenders the questions of how does one choose 'what to do' and 'how does one know what to choose'. The inscription above in the forecourt of Temple of Apollo at Delphi was "Know Thyself".
And that means what?
The great thing about being human is that we are all born different. Look at your preferences and qualities. What colour do you like? How tall are you? What attracts your attention? What books and subject matter attracts you? Asking simple questions like these will help you answer bigger questions. And there are no 'right' answers except the ones which are right for you. It's all about being authentic, and being authentic to no one but your self.
If who you find yourself drawn to chemistry, study chemistry, but half-way through studying chemistry there is a pull to study physics, then do that, because studying chemistry may have only been a way for you to find the deeper self-meaning that you truly prefer physics. And if you are going to be a physicist, be a physicist. Be an authentic physicist.
If you feel your self being pulled to believe in God and Jesus, then believe in God and Jesus. It is no one else's choice but yours. It is neither right nor wrong, it is merely what is you. So, be a Christian. Be an authentic Christian. Or an authentic Buddhist. Or an authentic Atheist. It doesn't matter what anyone else believes in, it is what you believe in which counts for your life because it is your life and you are the only one living your life, without any purpose or reason except the ones you choose.
And that's where responsibility come in. If you make your own choices, then you make your own consequences. That is what it means to be 'condemned to freedom'. You will never ever be completely sure that what you have chosen is 'the right way' because Existence offers no 'right way'. There is no absolute outside agency or 'markers' to compare your choices to which will confirm your decisions. The only 'right way' for your life is the way - or ways - you choose.
Obviously, there is more to this that just this, but this is the beginning.