Hmmm afew points.
You need more than just a doctor who prescribes you afew pills. Antidepressants can be part of the solution, however it's all a hell of alot more complicated than take afew happy pills and go on your way. Instead go and find a proper counsellor who will dig a hell of alot deeper as to what needs to change within your life.
Farminglad advice about John of Gods, thats not bad advice at all, very extreme jump for you right now, however the longer you stay on this path and the further down this depression rabbit hole you go the more you'll need to seriously consider it (and I'm hoping things won't spiral any worse for you, but trust me they could).
In terms of the farm, your being hard on yourself. And many of us dairyfarmer are. 12hour lonely days, doing many jobs we don't want to, in sh1te weather, financial pressures, stress, societal issues (farming getting a bad reputation with the public the min). But as a result, there is always milk on the shelves! We export alot of milk to China, a country who still has a significant rural working class, and their aim is to feed each child half a pint of milk, so a herd of 100 cows helps feed 2000 kids a day. And fcuk the environment or vegan argument here, whats their alternative solution, they have none, those kids remain longer in a nutritional deficiency than they need to, simple as is. And alongside all this, you have brought a daughter into the world, which is a wonderful thing, but also
a serious change and disruption to your life. So yep, your in a rough enough place podge and not at all enjoying dairying the min by the sound of it, but at least take some pride that you sound like a very competent and productive man, and you doing very useful work despite all the challenges.
Your immediate relationships. 2bh sounds like all of these need work. Your dad. Where do I start ha, because trust me have that t-shirt. You've signed over the land etc, but I'm guessing he still pulls many the strings in the business end. You aren't going to radically change him ever, but you can most certainly change the circumstances with him, suggestions I'd make are set alot better boundaries, the eveningtime milking beings a very important one, but also set boundaries like we are working alongside eachother for like 10hours every day, and irritable with eachother as a result, so aim to work independent of eachother for more hours of the day. Are you actually both needed in the parlour at the minute? If not then either try to get him outa the parlour some of the time, or you go off and do something else during milking. And that doesn't need to be one big immediate step, bring the cows in, get everything setup, and you walk away to change strip wire etc for 1st 5 rows, and come back then. Anyways next relationship. Your wife. You say she's patience but getting sick of you going on by now. 2bh you need to sit down and have a hell of alot more serious conversation with her about your whole situation. Does she know about the antidepressants? And finally about relationships, who else you got very close to you, who you can (and hopefully do) talk through all this about? That's what close friends are about, often they are just an ear to listen to, but someone you get stuff off your chest to about, and if they are good empathetic friends (which you defo need one or 2 close ones like that ha), then across time they can help you rationalise your whole situation and continuously check in that your improving all of it.
Ok final point. The bigger picture. Have you any sort of 1yr, 5yr hell even 10yr plan ha?? You got one daughter, any plans for more children? Your asking loads and loads of questions here about the future of your farm, many of them quite radical, stuff you can't immediately jump into. You need to think long and hard about all this, and plan out afew longer term goals, and work towards them. Sticking with the dairying is almost definitely the most straightforward in terms of the economics etc, but 12hour day's is absolutely not sustainable. However if your heart really not in it and you got a good alternative option (which I don't think you do have yet??), Then work towards getting out of dairying, life too short and your mental health cannot afford to stuffer anymore. Set afew targets with your immediate relationships, your time outside the farm, go book into some real counselling, and aim to slowly work your way out of this whole mess, best of luck and keep us updated across time here.