How to undo very screwed Git merge and restore branch (accidentally turrned main into another branch and deleted the original)

Question

The Problem

OK, so, I had two branches: main and session

In the main branch I developed the main code, in the session branch I developed code for a new session system.

Occasionally, I merged main into session to keep it up to date. ("Merge branch 'main' into session").

Today I felt the new session system was ready, and merged main into session so that it's on par with main, before merging session into main.

I finally merged session into main, was happy that this long in progress feature is done, and to really feel like I was done, I deleted the session branch.

Now, I didn't notice at the time, but it seems Git did fast-forward (or something like that), and the main branch has essentially become the session branch.

This means instead of having one nice merge commit seeing all the changes from session, they're all spread out, since it's now effectively the session branch. Not only is this terrible for historic purposes, but it also effectively prevents me from undoing this merge if the need arises.

What I tried

Git reflog: There are many sources online that share the trick with git reflog to restore a branch, however this does not seem to work for me, since the main branch has essentially become the session branch.

Undo the latest (merge) commit: This also does not work for me, since the latest commit is now "merge branch 'main' into session", so that too undoes all changes to main.

Further explanation

So, the last commit is not session branch stuff, but rather main branch stuff merged into main branch (since main has been turned into session, apparently).

Again, due to the regular merges and then a fast-forward, I essentially turned the main branch into the session branch. How can I undo this?

Apologies if I'm repeating myself, I'm just trying to describe as best as I can to make the issue clear.

Answer 1

Since main was now effectively session (fast-forward I think was the reason), I created a branch session from the top commit on main (since we're effectively on an equivalent of session due to the fast-forward).

Then on main I did git reset --hard HEAD^2 to get back to the "original" main branch before the screwed up merge (a fast-forward I believe).

So, main is again main, and session is at "Merge branch 'main' into session" (basically, what main just was before the reset). And this works because the parent commit is still there, just no fast-forward done yet (if I understood correctly).

I'm essentially back to two separate branches before the fast-forward.

Then I did force push main to GitHub, and set up tracking for session and pushed that too.

I do, of course, have all these ugly merge commits (from main → session) because I did not rebase, but that was an issue before this particular issue already and something I'll have to look into.

I hope this was understandable and will help someone in the same position.

Note: The reason "undo" failed on my first try was because I did git reset HARD~1 which went back just enough to undo all main commits in the merge without undoing the whole merge/fast-forward itself (if I understood correctly). Going back two steps did the trick.

Special thanks to @william-pursell and @LeGEC for the helpful comments.

PS It's difficult for me to put this mess into words, I'd gladly appreciate feedback on how to improve my questioning and answering.