In Jeremiah 12:7, who is the "Beloved of my Heart"? I have forsaken My house, I have abandoned My inheritance; I have given the beloved of My soul Into the hand of her enemies. - Jeremiah 12:7

This Scripture portion is one of Jeremiah's confessions to the Lord. He mentions a Beloved. On reading through a reference Bible, it was suggested that it might mean Benjamin, as mentioned in Moses' prayer about the tribes of Israel.  

Who or what exactly is the Beloved of my Soul Prophet Jeremiah mentions?
Contra your assertion that "This Scripture portion is one of Jeremiah's confessions to the Lord", most conservative commentators interpret a switch in voice starting from verse 5 (The NIV even inserts a section heading to that effect) - the prophet is recording the words of the Lord:

Ellicott's Commentary for English Readers - 


  ...The speaker is clearly Jehovah...


Cambridge Bible for Schools and Colleges - 


  ...Jehovah’s heritage has turned as a savage beast against Him. Therefore He has given her over to be the prey of neighbouring nations...


The Pulpit commentary - 


  ...Jehovah, not the prophet, is evidently the speaker...


In harmony with this perspective, the Beloved is generally identified as God's chosen people, the children of Israel, but which (particularly in immediate context), could be more specifically focused as the remnant of them extant at the time, ie Judah:

Matthew Henry - 


  ...God's people had been the dearly-beloved of his soul, precious in his sight, but they acted so, that he gave them up to their enemies...


Matthew Poole - 


  ...God by his house here understandeth the temple, which God is said here to have forsaken with respect to his gracious manifestations in it to the people that came thither to worship him. By his heritage he means the whole body of the Israelites..."I have given the dearly beloved of my soul into the hands of her enemies"; that is, he had given that nation which was once his dearly beloved into the hands or power of their enemies... 


The Pulpit commentary - 


  ..."I have forsaken mine house." The "house" is here not the temple, but the people of Israel, as the parallel clause shows...