Handling separation anxiety(?) or whiny-ness in 11mo

I'm really struggling with the extent of whining/crying from my 11mo at the moment. Things were difficult at the beginning but from about 4mo she started just making a slight whining noise almost constantly some days. It is as though she is actually distressed about something but can't communicate it. I went to the doctors who of course slightly wrote it off as though I didn't understand that babies cry.

It is still going on all these months later. When I spend extended time with my family they also express the concern that she does seem to complain or be distressed quite a lot. And judging from mothers group (who are very supportive) she is the baby most likely to have a break down at any given moment.

At various times I have tried to put it down to teething, separation anxiety (although of course often I am right there), developmental stages... But surely it shouldn't be a phase that lasts this long and continuously? Someone told me to look at the wonderweeks, but that just depressed my as either it is supposedly good week, or else she is just at the start of a bad phase and of course it actually never ends.

I guess that is confused by the fact that every now and then she has a good day or two, but it is very noticeable when she does! My worry was recently compounded by reading somewhere that a baby her age should be able to play by themselves for up to 40minutes!!!!(really?) I'd be amazed if she did 4 without expecting me to be right there. It's not that I expect her to amuse herself all day but even if she is happily playing for a minute or two I usually can't walk to the kitchen (where she CAN see me) without her coming and weeping at my feet immediately.

The last straw is that daycare also admit they are little concerned at the extent to which she whines, even if she has had good sleep, feed and is clean/dry and being given some attention. I mean, they see lots of babies, so I feel like that means she is not quite in the normal range. They even asked me today whether I have considered discussing it with a paediatrician. I feel like I need to apologise for her whiny-ness when I pick her up each day (3 days/week). They wondered whether it was to do with daycare but to be honest it sounds exactly the same as at home. She's been there for 3 months already.

I just don't know where to turn. I don't know whether I'd be more concerned if this is quite normal (in which case I am really NOT coping), or if I did find out something was actually wrong either physically or emotionally. I don't know how to parent it as often the issue is that she wants me close by, but does not necessarily respond well to being held/cuddled.

Assuming it is mostly separation anxiety, how do I teach her over time to cope with me being on the other side of the room or not giving her attention for a few minutes, let alone leave the room? I try to sit quietly and not intervene too much if she is playing by herself, in the hope that she'll learn to do it for longer. But I also sometimes jump in and give attention even if she is happy, so that she doesn't think she has to cry to get attention. What else can I do?

I also have massive guilt (still) for breastfeeding issues which led to mix feeding and early weaning. And sometimes I wonder if she would be different if I had managed to breast feed. :(

When I hear mothers talking about how hard it is but it is 'worth it' I feel like they all must be lying/delusional or I am really getting it wrong. I love my daughter, but almost every day I feel like it was the biggest mistake of my life.

Sorry for all the info. Just hoping for other stories about how to get through this period.