@CroccyCroc she won't be able to demand to see DC as it suits if you get a court order about contact arrangements, but no one can actually make her take the DC at the times she commits to or to have them more than she wants. She wouldn't be the first person to fight for contact in court and then only keep it up for a short time. What outcome do you want ideally? Is it she has them EOW and pays CM? Or 50/50 and no CM or something inbetween? She doesn't have any more 'rights' to the DC then you do. Though it's not actually rights, you both have the same parental responsibility, she can decide not to see them ever, but if she just took them you could go court and make an official agreement on contact.
Given you say the same kind of issues occurred in your marriage it doesn't seem realistic to expect her to treat you better in seperation then she treated you when married. You're not going to convince her to stop messing you around or to consider your mental health needs. You stick up for yourself by following the legal processes to make contact official by going to court, you apply for child maintenance if she works, you get child benefit for all DC in your name and see if you qualify for UC and if you qualify for reduced child care costs. Try the entitled to website for information on what benefits you qualify for and I've heard wikivorce is good for UK divorce advice divorce.wikivorce.com/
Then like other parents who ends up with majority care with an unreliable Ex you find a way to make things work that isn't reliant on said Ex, who is only interested in seeing her DC when and if it suits her. I'm not saying that to hurt you and I expect it probably sounds impossible to you, but it's less impossible than your Ex suddenly being respectful and considerate of you and mutual DCs needs. It's the unfortunate reality for those of us who have DC with people like your Ex and my stbxh. You can't rely on her, you can't count on her, she doesn't have a better side you can appeal too and if she didn't respect you and consider your needs during marriage, there is no way she's going to treat you better in divorce.
I have SEN DC, an emotionally abusive, controlling stbxh who can't deal with our DC SEN needs and am disabled and very sick with a chronic illness, so I'm coming at this as someone whose very much had to force myself moment by moment to go on for my DC. All they have is me. You centre your DC in this and you find a way through, for all your sakes.
I know several amazing single parents, both female and male, who all have 90-100% care of DC. It's so far from easy, they all have various challenges on top of being the only responsible parent their DC have, various abusive Ex's, SEN, their own life limiting illnesses, severe mental health issues and I know how hard they find it, we have our breakdowns when things get too much, cry, sob, think we can't go on, rail at life and then force ourselves to get back up again and go on as best we can. Those struggling with mental health have stuck up for themselves by seeing their GP, taking medication, trying new ones if those don't work, getting therapy, hanging on in the meantime because the waits can be long. We get back up again, because the alternative is our DC falling through the cracks.