External Diff Tool does not launch and files get stuck in Merge.

[Sarah Montoro]

I've seen many posts about external diff tool not launching but I haven't been able to find a solution that works for me. While I have some experience with Git from years ago, Sourcetree is new to me and I'm not that familiar with the command line, so the solutions I've found so far have confusing and I'm not sure if I'm implementing them correctly. 

Basically what happens is I try to resolve a conflict by launching the external diff tool, but rather than launching the file seems to automatically start a merge that then gets stuck. The little warning sign next to the file disappears and when I try to Pull or Push, I get the following error, which forces me to commit the merge before moving forward: 

error: You have not concluded your merge (MERGE_HEAD exists).

hint: Please, commit your changes before merging.

fatal: Exiting because of unfinished merge.

Completed with errors, see above