I'm unable to see previous messages I sent on my iphone on my Mac

Make sure "Text Message Forwarding" is ON.

On your iPhone, go to:

Settings

Messages

Text Message Forwarding

Turn your iMac ON

Also, on your Mac

Open Messages

In the toolbar, click "Messages"

On the drop down bar, select Settings

Select the "iMessage" Tab

Check "Enable Messages in iCloud"

Make sure that you can be reached at your phone number AND AppleID email address as well.

At the bottom of that window, double check that you're "Starting New Conversations From" your phone number.