Bifocal problems- do i need them?

I am and 17 and my optician gave me bifocals to use alongside my contact lenses because he said my long distance prescription would strain my eyes too much if i am doing close up things (like studying). I got them today and i HATE them! The line is blurry and just annoying but worse than that it feels like i have the worst sea sickness.
I'm not even convinced that i need them because i can go all day wearing my contacts whilst studding without my eyes hurting, but my optician seemed pretty adamant that i needed them and i'm not sure if he will write me another voucher (NHS) if i ask for the single lenses glasses. What should I do?

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