Hey LR, it sounds like you and your W have become closer and that is good.
Yes, be cautious about ever owning ALL the blame. Take a look and see if it was simply a tactic to win her back. If so, it may backfire on you and it isn't truly helpful to you nor your W. She would need to take her share of the responsibility in the breakdown.
You indicated that you were aware of the OM. Her being "sad and lonely" and you going back to he is likely an artefact of the breakdown of that R. I'm sure you know that.
Again, if you went back to her, knowing that was likely the case. I hope you weren't going back with the intention of being her knight. It is possible that she's leaning back on you, waiting for OM to change his mind...
In the end, this is all up to you. What you have done, appears to have worked... but is how you did it, or your intentions in it, something that you will respect yourself for... now or down the road...
Or...
Would you rather her come back to the M because she WANTS to be back in the M...
Did she miss YOU... or was it just that she missed companionship (and sex) so decided to settle for you while something better came along...
I hope that you can keep the connection going and growing with your W... but what can you do to give her some more space and time to think what life would be like, with the NEW, and AMAZINGLY improved LR...?