Your inbox can fill up very easily, that's why Gmail has a variety of tools that you can use to organize your emails, and if you are looking for a specific email, especially if you received it a long time ago, try using the Search functions by date, this will allow you to see every email you received on a particular date or between two different dates.
If you generally know when the email you're trying to find was sent, there are two ways to find it.
Search Options Menu: You can use the search options menu. Clicking the icon that looks like a string of keys in the Gmail search bar will open a menu that you can use to select the type of email you want to see. One of these options allows you to date under Choose a date and see every email you've received within a day, week, month, or year of that date. This is good for finding a wider range of emails over a longer period of time, and you can use the other menu options to narrow your search even further. But if you want more control over the dates you see, try using keywords to search by date.
Use keywords: Typing certain keywords into the Gmail search bar will let you see results you wouldn't otherwise get, and all of these things work in both the mobile app and the desktop site.
If you are searching for emails from before a specific date, use the format before YYYY / MM / DD, and to find emails after a specific date, use after YYYY / MM / DD.