Hiya sweetie....
I know, it's hell, isn't it? I'm a fair bit younger than you and 27 weeks pregnant, but in a v similar situation: my ex cheated on me throughout our relationship (although the pregnancy WAS planned and his idea!) and is now with the teenager he cheated on me with....though I know he has slept with quite a few random girls since we officially split 2 months ago. It hurts like crazy...&, like you, we were seeing each other still once a week in a attempt to "stay amicable" and he was helping me get my house all set up for the baby etc. But whenever we argued, and esp now that he's out in the open with his mini gf lol, he just keeps threatening me with solicitors and shared residency.
In absolute truth, maintaining contact weekly was poss the biggest mistake I was making, because I just couldn't have any chance to get over him, to step back & see what an idiot he was. When you're alone, lonely & feeling krappy, I think you come to "rely" on that once a week visit, especially when it's offering a kind of support like helping with your home....but that's really the worst kind of dependency to be maintaining. My ex has just been on holiday for two weeks, & it's been kind of a relief not seeing him. Now just gotta stop imagining him with HER....& worst of all thinking of her in my baby's life.
Cos the sad fact of the matter is that, legally, your ex is dead right....if there's no doubt he is the biological father, you CAN'T stop him seeing his child & being involved. And I'd be really really careful about sending him narky emails threatening to, however hurt & angry you might be feeling....I only offer this advice because my ex has already started the court proceedings etc before our LO has even emerged (ridiculous, I know) & any emails like that you use will potentially be used against you in court to make you out as a "contact blocker" and strengthen his case. The law in this country doesn't give a damn if a partner walks out on his kid (or hers to be fair) for another woman, or if he cheated during the relationship. In fact, my solicitor has warned me that if you get CAFCASS sent round, to attempt to bring the cheating etc up will likely backfire in you because it makes you look bitter & like you're trying to hurt your ex, not acting in the best interest of your child. Believe me, I know how hard this is....I feel exactly the same. BUT he has every right to not only be involved with his child, but under the UK legal system he can also have any woman he's seeing (unless you can prove she's a threat) involved with the child too.
So all people like you & I who have been utterly screwed over can do...is try to rebuild our lives alone, forget about the ***s who put us in this position & if at all possible make things as amicable as we can possibly bear to (without holding any hope of getting back together!), because otherwise all that lies ahead is Court and tears. Not nice.
Good luck!!!! (Reckon we all need it...or a flippin' sainthood if we pull it off!)
xxx