I've given you my word and I’m sure to help you.

我答应过你，我肯定会帮你的。

Give you my word = I promise you; You have my word; I give you my word; I swear to you. 我保证 / 一言为定。

例句：I give you my word (of honor) I won't tell anyone.（我保证不告诉别人。）

I'm sure…：我肯定……

例句：I am sure that you will understand. （我肯定你会理解。）