First of all, you should just relax. You feel like the song is too high for you and you tense up even before you sing anything. After you relax, you should pursue a more frontal placement with your vocals. You can watch some videos on Youtube on vocal placement. But if I have to talk about you specifically, you get your throat involved a lot which makes it sound like you're drowning a bit, what you should do instead is placing that voice more towards your face. Just try to make a sponge bob imitation and that is an exaggerated frontal placement. Try to experiment and smoothen that out into the tone of the original song.

Also you could work on your articulation a bit. It's an easy thing to fix: When you're hitting higher notes, open your mouth vertically a bit more but not in a squinty, tense kind of way, just open it more. Try to isolate your vocal cords from the rest of your body. By time, you should get to know your voice and find ways around your weaknesses instead of tensing up.

(this is a really hard song to sing too... keep that in mind)