I realize this thread is over 4 years old, but it was perfectly applicable for my troubles with my first mortise and tenon. The comments made in this thread were very helpful.
Below were my keys to success on my fourth attempt:
1.I re-sharpened my chisel.
2.I cleaned out the mortise with a drill bit at the drill press from both sides.
3.I clamped a guide on the long side of the mortise so I had something to register my chisel against.
4.I did one side at a time working slowly back and forth down the side with the edge guide until I was halfway through.
5. I exercised some patience which is hard for me personally especially on the short sides of the mortise where it seemed to be pulling chunks out before.
The words of encouragement to my fellow new woodworker were very helpful for me as well. I was frustrated to say the least. I felt reviving this conversation might help someone else struggling in the near future.
I’m still struggling with the length of mortise being to large for my rails causing a gap when at the top and bottom. Not sure what I’m doing wrong yet, but I have confidence I can figure out what’s causing it and find a solution that works for me.