I think you need to compromise if possible. Your DH doesn't have the option of going part time because he's the higher earner, but he still has a right to be involved in the decisions about his child. Maybe he thinks full time nursery isn't the best thing for baby at this age? Ultimately it's your decision but it's only fair to consider his feelings and opinions.
I was keen to return to work after mat leave but we compromised and I went back 3 days a week. I found long days with a baby sad and depressing too. Getting treatment for PND really helped me. Once baby turned 1 and started walking/talking everything changed and I started to really enjoy him. I found the baby months a drag but love having a toddler!
As for structure, I think that's something you can create if you want to. Once they can play independently, groups become about them not you. Try lots of different groups, make friends, join FB meet ups, plan trips to zoo, softplay, farms, swimming, structure in educational sessions, work on language skills etc. I find it helpful to have a plan for the days I'm not working, I write it down the night before.
I love my job, but on days I work I feel like everything is a rush- getting to nursery on time, commuting, nursery pick-up, keeping on top of housework and laundry. DH works long hours so most of the household
things fall to me. Just trying to blowdry my hair and get dressed for work is a struggle when trying to get baby ready for nursery!
If you return full time will your DH take an equal share of household chores? Can you afford a cleaner/housekeeper? Who will do the cooking? Who will look after baby when she's ill? You need to discuss all this before making the final decision.
Also, if you change your mind or find full-time too much, is there an option to drop your hours later or are you commited to full time once you return?