主句是keep sth in mind 结构。what从句为宾语。这是名词性关系分句。但what从句结构有误，what在从句中无成分可作了。可修改如下：

I'll keep what you have helped me with in mind forever.

I'll keep what you have helped me to do in mind forever.

最好采用宾语后移：

I'll keep in mind forever what you have helped me with/to do.