Xcode Certificate problem

Please, I absolutely need someone to help me out

I've been stuck for a week and I'm about to throw my Mac off the balcony.

Last week I finished (finally) an App of mine and I started to upload it to the App Store: from there the trouble started.


I started having problems with absurd password requests and I thought it was an XCode update problem, so I started downloading the upload of over 2Gb (and here the fiber is not there ... the connection is about 4Mbs).

At the end of the download, nothing happened, so I repeated the operation 5/6 times, for a couple of days, without result.

I removed XCode then and reinstalled from zero, and finally all was well,

Now: nothing works anymore: in practice, it no longer allows me to install my Apps on the Device nor to create Builds for the App Store, but in the simulator everything is fine.

I think it's a problem related to certificates, but I don't know how to get out.

Can anyone help me?

I am desperate: I'm not very able to manage certificates and I've tryied to look on the Forums but there's something wrong.