どうしても is a construction in Japanese that is often considered to be a stand-alone 副詞（ふくし） (adverb) meaning ‘no matter how’, or ‘regardless of anything’. Originally, it is a combination of the 副詞（ふくし） (adverb) どう ‘how’, and the ても form of する, しても ‘even doing’.
どうしても will regularly be used at the beginning of sentences before conveying something that is considered by the speaker to be ‘absolutely true’.

• どうしても欲（ほ）しいのなら、自分（じぶん）で買（か）いなさい。
  If you want it no matter what, buy it yourself.
• どうしても我慢（がまん）ができなくて買（か）っちゃった。
  I accidentally bought it because I couldn't resist no matter what.

Alternatively, どうしても may be used after verbs, following the 格助詞（かくじょし） (case marking particle) と. This will express that ‘if (A) happens, (B) is unavoidable’.

• 牛乳（ぎゅうにゅう）飲（の）むとどうしてもお腹（なか）が痛（いた）くなるんです。
  Regardless of anything, If I drink milk, I get a stomach ache.
• 寝（ね）る前（まえ）にお腹（なか）いっぱいに食（た）べるとどうしても寝（ね）れなくなっちゃうんだよね。
  When I eat till I'm full, before I sleep, I am unable to sleep no matter what.