Monkold – I’ve drafted a redrafted a response to you three times and deleted it each time! Your situation is complex, and ironically is going to motivate me to post my own question later today – on something I’ve been mulling over for a week or so.
The simple facts are as follows...
Yes your skill set is excellent for contracting.
Yes you could earn more in the short term
Yes you could also move to another firm, but it would take a lot of fortune to get in at SM, having missed it at you own firm, while moving at M would simply require starting your promotion campaign from scratch.
I have nagging question as to why your mentor changed half way through the year and the way you managed that? You have also realised that presentation is 80% of the case for promotion at most consultancy firms – yup, it’s a campaign.
On the face of it your CV is perfect for senior progression within a big4. It would be a shame to lose out just because of a tactical set-back. Frankly a lot depends on whether you feel you have what it takes to make it to the top, or thereabouts.
One thing you should do is recognise that this is an excellent time to assess your options and a terrible time to make any irrevocable decisions. WRITE down a page or two about how you feel. Get those close to you to provide some of the input. It will make an interesting read in 90 days and whichever route you look to go down is likely to take at least that long to put into place. If you still feel as “crushed” then, it will strengthen your resolve. If it all looks like a storm in a teacup, it will ingrain a different lesson.
I’m sorry I can’t be more prescriptive, but your situation deserves more than generic platitudes. One thing I will say is cheer up. Regroup, move sideways, got to industry or contracting? All excellent well-paid options – and you’re in a fantastic position to choose from any of the four. Don't confuse having to make a decision with being in a bad place.