The thing about mediation is that it's supposed to be a way for you to reach an agreement of your own accord, rather than having other people decide for you (such as Legal Aid or the judge).
You're on a more balanced playing field, however, if Legal Aid is conducting the mediation because they will take care of the details for you, such as where it's being held and making contact with your ex. Basically, there's no games about wrong addresses and what not.
There's not a great lot of legal advice that can be provided for mediation because it's supposed to be a non-legal-focused resolution avenue, but Legal Aid ordinarily appoints a solicitor to provide support to you.
I can provide personal experience, though.
My partner and his ex had two mediations - one before he initiated court proceedings and one during proceedings under orders. Both failed because the mother was preoccupied with non-issues, like who drove the car when the child was in it and whether communication was via text or e-mail. The mediator got so bogged down in the unimportant details that the core problem, which at the time was how much time the child was spending with the father, hardly got a shoe-in.
However, like you, the first mediation was especially terrible. The father was pressured into a signing a parenting plan he didn't agree with and he felt like nobody seemed to want to focus on the main problem. They just 'humoured' the other party until time ran out and discussion about the biggest issue got only a minute or two air time.
In his second, it was much better. He was more prepared and much firmer about his approach. He was also self-represented, so rather than depending on someone else to tell him fragments of information about the legal principles, he knew them for himself, which have him a lot of confidence and grounds for making his points.
So, my first line of experience is to write a list of the core issues. In your circumstances, I would say it's who has parental responsibility for your oldest daughter, and how often you see your youngest daughter. If you want to also include a list of secondary issues, like how you'll communicate with your youngest, I would suggest doing that also.
What also helped my partner stay the course in mediation rather than have it overrun with unimportant non-issues, was to also write what he saw to be the problems with those core issues under current circumstances, and why the solutions he proposed would be effective. The list was short and precise, unemotional and always about the child, never about the other parent.
I think keeping the session child-focused is also very important. Things like post-natal depression, arguments between you and your ex, new relationships, they're not variables and they have little to do with the children, so don't give them air time. They're usually only brought up to try and intimidate you or because they see mediation as a forum for airing all their criticisms about the other party, which doesn't help resolve the problems. So, if those things are raised, you can say polite things like "I would like to keep this mediation focused on the children's care arrangements, please" or "We are here to talk about the children, not my medical history, so please can we remain focused on what's best for the kids".
Doing this requires you to remain calm and assertive. Being assertive means saying things like, "I think we could better meet (x)'s needs if I had parental responsibility so that I could take her to a counsellor" rather than "You are making (x) suffer because you won't let me take her to a counsellor". Using "I think..." or "I feel..." or "It is my view that..." is less aggressive than using "You always..." or "You never...", and the less conflict there is, the more likely an outcome will be.
And finally, don't sign anything if you don't agree with it. My partner initiated proceedings because he signed, on legal advice, a parenting plan that he didn't agree with and the first thing the judge did was put the plan in as interim orders because the plan seemed to be the only proof the judge had that the parents had agreed on something. In the second mediation, he was more wary, and refused to sign anything until he had received legal advice, and the other party was reprimanded harshly for trying to demand that he sign on the spot.
Anyway, I hope this helps, and good luck.