I apologize that I wasn't specific in my first response.
Yes, I did try everything you suggested, but it is still the same. Emails I sent a month or two ago are there, along with everything prior, but anything I have sent (created) or emails I have replied to, only stay in the Sent box for a short time. A few hours.
When I say I "tried Everything" I can think of, I mean I looked at all the help responses in Mail and GMail and could not find an answer. But I see others have the problem.
I have gone to Settings - Accounts - Mailbox Behaviors, but I have not found anything that says something like ...Sent mail goes to Sent folder, or Sent mail will remain in Sent Mail folder for X amount of days.
I use both Apple Mail and GMail and have for years. This was never a problem and I only noticed it when I went looking for an email to confirm that I had sent it. The folder only contained emails that were sent before early February.
I am reluctant to reinstall the Mail app and the OS because I fear I could lose something very important.
My Mac OS is up to date and so is my IPhone.