I'm slightly confused but I'm raising this as the line of questions raises the risk that you have a pretty poor settlement agreement with your ex and while you might not be able to change it, you shouldn't be doing him any favours.
You are selling up now because you can't afford the mortgage? Is this 100% of the mortgage in that you have taken on the full cost of the payments when your ex jumped ship? In which case, he should not have 50:50 equity? Or does he still make payments but your financial situation has changed and he is seeking to leverage the situation to get his equity two years early?
If it's the latter, he still has an obligation to fund his daughter and assuming cannot or will not, you should be seeking to do that as part of a bigger share of the equity which might give you more choices.
An alternate is that your daughter boards with a family during the week and he/you jointly funds that.
IF YOU DON'T want to move, as you haven't started to market the house yet and a move might take 6 months in the current market, I'd be trying to hang on for the next two years so that you are moving house in July 27 after her A levels.
I would speak to your solicitor to establish your rights and options and then your bank to see what they can do to help. Your home appears to be in an upmarket area with easy sale potential so less risky for them. Banks don't want to repossess and evict families it looks very bad on the front of the Daily Mail. Far less disruptive to see if you can take a mortgage holiday or reduce your payments temporarily.
IF YOU DO want to move, have a clean break from your husband then assuming the commute is a straightforward train journey then I would just crack on with it all. Your daughter will adjust and while 2 hours on the train is inducing gasps of horror it can be a nice way to read a book, watch a bit of tv and unwind after a long day.