i have a 100 page document that i am still writing. i made an index in the beginning and as it gets longer and longer it is harder to find the text i want

is it possible to link the index text in the beginning to each same heading that occurs later in the text? if so how?

Ordinarily, one would use Headings and a Table of Contents at the font of a document and an Index at the rear. By default, a Table of Contents hyperlinks to the corresponding Heading. If you add the hyperlinks Forward/Back buttons to the QAT, you can use them to quickly move back & forth between the TOC and the headings. If you want to cross-reference a Heading, you can do that via Insert|Cross Reference. Indexes in Word don't support hyperlinking.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]

How to create a table of contents in Microsoft Word by Shauna Kelly
Why use Microsoft Word’s built-in heading styles? by Shauna Kelly

Paul is right. The simple solution is to use the TOC feature. It has built-in hyperlinks and if you use the build-in heading styles, you do not need to specially mark anything.