どうしても
no matter what; just (can't)
どうしても + たい / なければならない · どうしても + negative
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どうしても has two sides.
With a wish or an obligation, it means "no matter what, at all costs": どうしても日本に留学したい, "I really want to study in Japan, whatever it takes"; どうしても今日中に終わらせなければならない.
With a negative, it means "just can't, however hard I try": この問題がどうしてもわからない, "I just can't understand this problem"; 彼の名前がどうしても思い出せない.
It's the fixed form of どう + しても, "whatever I do". どうしても〜と言うなら means "if you insist on".
Don't confuse it with どうやって ("how, by what means") or どうも ("somehow, really"). In a grammar question, look at the ending: a wish or a must means "no matter what", a negative means "just can't".
Examples
どうしても日本に留学したい。
I really want to study in Japan, no matter what.
この問題がどうしてもわからない。
I just can't understand this problem, however hard I try.
どうしても今日中に終わらせなければならない。
I absolutely have to finish it today.
彼の名前がどうしても思い出せない。
I just can't remember his name.
どうしても行くと言うなら、止めません。
If you insist on going, I won't stop you.