This sounds very familiar. I had my first (and only) cutaway on my 13th jump (which also happened to be my low solo). I also failed to pull my reserve handle.
My main appeared to open fine. When I released the brakes, only the right brake released and then I started to spin very fast. I tugged as hard as I could and couldn't release the left brake. I looked up at the lines and couldn't see any problem. I also had very little luck slowing down the spiral, but I'm sure a more experienced jumper could have done a better job. I just knew I was losing altitude fast and I opened lower than usual anyway (because it was my low solo) so I didn't waste a whole lot of time before deciding to chop it.
I looked at my cutaway handle, put both hands on it, looked at my d ring and pulled. Maybe because I was spinning, or maybe it is just the way it always happens, but the change in attitude when the main released caused me to momentarily lose site of the d ring. I reached for it again and by the time I had my hand on it, the reserve was already open. I therefore didn't pull the d ring. I landed uneventfully and then got the same admonition from the instructors that once you start pulling handles, you don't stop, no matter what. I know I would complete the sequence if the same thing happened again.
Unlike your malfunction, I still don't know what the issue with mine was because when we found the main after I chopped it, the left brake was free. Someone suggested a tension knot, but I have no idea.
As one of my instructors said to me...You did great as you got down safely. Experience matters, and you are now probably better prepared than many jumpers who have far more jumps that you but who have never experienced a cutaway.
Believe it or not, I felt alot more comfortable after the experience because then I knew I could handle a cutaway when the time came.
Blue skies.
