Apache rewite or alias- remove part of url

I’m setting up a reverse proxy and this is the first time working with mod_rewrite. After some long research, I believe rewrite or alias is the trick to resolve my problem.

Apache Config
ProxyPass /site123/ https://x.x.x.x:8443/portalapp/
ProxyPassReverse /site123/ https://x.x.x.x:8443/portalapp/

This portion is working okay, except when a user clicks on a button, the url shows as: /portalapp/controller?handler=LoginHandler&action=noaction (embeded html code)

When that happens, the new URL in the browser ends up as https://x.x.x.x/portalapp/controller?handler=LoginHandler&action=noaction

I’m trying to get it to show up as: https://x.x.x.x/site123/controller?handler=LoginHandler&action=noaction

I’m trying to use Rewrite to look for the /portalapp/ and remove it. Maybe alias is a better route to take?

Any suggestions?

Thank you