Winnie, yes I've found it hard adjusting to having a second child, too. Can really sympathise with the complex childcare situation you are facing.
I have a nearly 7 year old and a 20 month toddler. When I returned to full-time work, it took loads of planning to find a system that allowed us to get both sons seamlessly to school/ childminder/ after school club etc. If I'd been on my own all this to-ing and fro-ing would have been extremely daunting. And then there's double the dentists appointments/ double the potential to have to cope with a suddenly ill child as you're about to dash off to work in the morning etc etc etc. We'd just moved to a new area and to cope with this I deliberately created a geographically close network of support: our new family doctors, dentists, school, childminders etc are within a 5-minute drive of each other. This really helps a lot: When I pick up my toddler from the childminder, our doctor's surgery is just round the corner, so if needed,( with a bit of sweet-talking the doctor's receptionist) I can immediately pop him in for a check-over, just before closing time.
And, this may apply to you even more than me, my eldest son is now getting so independent that it felt like I was beginning to get my own life back. To then go right back to square one with a new baby seems very hard. There's so much I can now do with my eldest now he's no longer the attention-seeking, exhuberent 4-year old that he was. I can really relax with him at home and when we go out I don't have to watch his every step. He's good company, funny and helpful. Then, just as it was becoming commonplace to take him to cinemas and restaurants, have saturday morning lie-ins, to arrange the house as I want it, to do gardening and decorating again with ease, son number two comes along, and it's back to pushchairs, nappies, fridge-locks and outings to the toddlers section of the playground in freezing February!! While I'm seeing some of my friends who have only one older child, enjoying encreasing amounts of freedom. Winnie, it's difficult, however much you want that second child.
At least you know that with your youngest, everything really is a phase, they will eat with a knife and fork, get potty-trained - eventually. You've already seen it happen. And you don't have to learn all that practical parenting stuff from scratch. And they are not old enough to have too much ' attitude' or want expensive toys and clothes.
With an 11-year-old daughter and a partner, you've got help at hand this time round, too. I'd try and involve them as much as possible in the day-to-day stuff, like doing the evening washing up, or doing some of the running around and collecting from childminders, school etc , especially if you are going back to work. If they can take full responsibility for set tasks, then it's one less worry for you. You've probably already done this anyway.
I too felt I had a mountain to climb before I returned to work the second time round. I guess I knew exactly what I was in for, having been there once already! But you've already bought up one child single-handed,studied and worked, and that must have really challenged your powers of organisation at times. I obviously can't say whether its right for you to go back to work or not, but it seems to me that you can cope very well with lots going on in your life. Good luck!