Remove part of URL - rewrite/alias?

[3c2controller]

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 mod_alias is a better path to take?

Any suggestions?

Thank you