TFS : Discard un-checked merge changes

Question

I tried to find answer for this but could not find. I am sorry if you feel it is duplicate. Please point me in correct direction.

I did a merge from Main to Source. Due to some internal issues and build problems we decided to merge QA to Source . I did the following steps

1) Deleted my Source folder

2) Open Source Explorer via Team Explorer in VS 2012 and get latest code into my Source Code.

3) Open Source folder in Windows Explorer to see that I have all the code which I want.

I open ALL.sln inside one of the Source folder in VS 2012. I noticed that in Pending Changes I have 14125 changes. These are changes that happened when I did merge from Main to Source. I tried to undo these pending changes and every time my VS 2012 hangs and after 20+ minutes I get Internal Error. TFS server not found.

My question is how I can discard Merge Main to Source changes and start my task of Merge QA to Source from scratch.