Sorry you’re feeling like this and you don’t have a supportive manager. That’s the last thing you need.
You have been to a doctor who has decided you are not fit for work at the moment. This is fine, I know it’s hard (esp. with mental health struggles) not to feel guilty or feel a burden to everyone at work but you need to take care of your health. If you had broken both legs you’d not be feeling as guilty I bet, this is the same thing. You need this time to get well.
When you are in the meeting, as it’s informal (totally normal when you first go off) they will likely ask how you are doing and when you think you might come back. If you don’t know DO NOT BE AFRAID TO SAY THIS! Just tell them it’s too early to say. They might ask if you’ve been prescribed any treatment, medication or counselling. If you haven’t, don’t worry it doesn’t make it any less believable. If you don’t feel comfortable answering a question, say so.
The GP and your occupational health department will be the lead on you going back when you do eventually go back so don’t feel pressured into doing it earlier than you’re ready. Ultimately, you’ll set yourself back. Again, broken leg analogy, you wouldn’t go on a ten mile hike 4 weeks after breaking your leg, you need to look after yourself.
A couple of points for you, when you are thinking you might be ready to go back, you need to make sure they do an occupational health referral for you, which should be done for long term sickness (that is any absence over 28 days) OHU are there to support you and can recommend reduced hours temporarily (on no less pay) or other adjustments to help you settle back in.
Also - and this is important - You are protected under the Equality Act for 18 months after you give birth. Remember this, in case they try anything unreasonable.