The chocolate I intended to give you as a present (souvenir) melted so I had to throw it away.
Unfortunately, the chocolate I bought you as a souvenir ended up melting so I threw it away.
「あなたにあげるはずだったお土産のチョコ溶けて捨てた」= The chocolate I intended to give you as a present (souvenir) melted so I had to throw it away. / Unfortunately, the chocolate I bought you as a souvenir ended up melting so I threw it away.
ボキャブラリー
intend = ~をするつもり
give = 渡す、プレゼントする
present/souvenir = お土産
melt = 溶ける
have to 動詞 = ~をする必要がある
throw ~ away = 捨てる
unfortunately = 残念ながら
ended up ~ing = ~してしまった
※「souvenir」は「旅行などに行った時に記念するために買うプレゼント」というニュアンスになります。旅行中に勝ってないお土産の場合、「present」という単語を使った方がいい。