Your multiple-entry visa is issued by Italy, but not exclusively "for" Italy.
It is valid for the entire Schengen area, and you're perfectly entitled to make short visits to Germany (or any other Schengen state), especially when you have already used if for the Italy trip you described in your original application.
Here "short visits" means anything that complies with the general 90/180 day rule, so your planned 6-week visit is still completely good for your current visa. It also doesn't matter whether you're traveling to Germany directly or through another Schengen state.
The entire point of issuing a visa with long validity and multiple entries is that you've now been vetted enough to cover all subsequent short visits to anywhere in the Schengen area for as long as the visa is valid for. Part of the Schengen arrangement is that member states such as Germany and Italy trust each other to do such vetting on each other's behalf.
In fact you can't get a German short-stay visa as long as the Italian one is valid -- because it would not allow you to do anything that the existing visa doesn't already allow you to do.
In principle you can ask to have your Italian visa revoked and then apply for a German one from scratch -- but because that would be utterly pointless, it's likely to raise suspicion that you're trying some kind of hustle that the consular personnel cannot figure out, and your application might be treated with extraordinary scrutiny, risking a refusal. So don't do that!
You're probably being confused the rules about main destinations. These rule govern which Schengen state can process a given visa application. Once you have managed to get a visa application processed (and your claims in that application were truthful), those rules are irrelevant to you. There's no rule saying that each trip you make has to be covered by a visa issued by the main destination for that trip. Only for trips that you want to use as reason for applying to a visa does the rule kick in and tell you where to send that application. But in this case you don't need to detail your Germany trip in any visa application, so it is immaterial where you would have sent that application if you need to get a new visa for the trip.
TL;DR Your existing visa is all you need. Do not apply for another one!
