First things first... Drop the expectation that you can just try and live in the present moment all of the time and it will just happen. There are very few people, if any, who can maintain present moment living on a permanent basis.
However that doesn't mean that you need to give up on practicing living in the moment, on the contrary, stick with the practice and keep at it. Yes, there will be times when the worries or thoughts win over, but recognising that they have done that is the first step and you can use that as an indicator to yourself to say "oh, hang on, I need to be present again", and then bring yourself back to the present moment.
It takes practice, and the more you start recognising when you are not living in the present moment, the more you can start bringing yourself back to it and the easier it will become.
I assume you've been to see your doctor about the nausea and dizziness from being hit in the go cart? It could be a whiplash type injury causing a trapped nerve or something, but you definitely need to pester your doctor to get it looked into and get a proper diagnosis. However, if you spend all of your time worrying about tommorrow, then what are you doing today? And what will you be doing tommorrow apart from worrying about tommorrows tommorrow? Surely you can see that logically there is absolutely no point in worrying about tommorrow. What happens then will happen, and it is Now that matters in the present moment.
Hope you get things sorted ok.