Sorry to come late to your party!
@JC, Using hosting with GoDaddy is your first error. They are notorious for being a terrible host (albeit a good registrar). There are so many posts here complaining about GoDaddy that it's surprising to find members still being aboused by them.
Hosting with unlimited anything is (a) a misnomer (because there is no such thing) and (b) an indication that you're using shared hosting (and a shared IP Address - which cannot be used with a secure server certificate).
Secure server certificates are not cheap! Multi-domains are even more expensive. Of course, you bought from GD so they're outside GD's core business and likely not to do anything useful but allow you to use encryption. Personally, I "share" my own secure server certificate with clients as a cost-saving measure for them (both the cert and required IP Address are yearly charges) with the only downside that the URL for their using my cert includes my domain and their directory in the path to their secure file(s).
Now, it appears that you want the entire websites encrypted (in transmission): Why? That seems to be an abuse of the secure server as it's normally reserved for handling sensitive information. For example, look at ZenCart's parsing of links into secure and non-secure handling.
SS directed you to the mod_rewrite tutorial linked in my signature where I have mod_rewrite code to direct to https in a universal manner as well as selectively (which I recommend). Did you bother to read any of that? :nono:
An Internal Server Error is normally the sign that you've had a syntax error in your .htaccess file. Check that carefully and, if you can't spot the error, comment out the entire file then uncomment a line at a time to discover the culprit.
Partial encryption says that you've included files which are not encrypted. To me, that says that you're using absolute links within the scripts loaded which are http://yadda.yadda/unsecured. Have a look through your code for something like that.
You might benefit from reading the mod_rewrite tutorial linked in my signature as it contains explanations and sample code. It's helped may members and should help you, too.