Well, although Liz's advice is sound, I doubt you will be able to get your tiel to eat fruits -at least, none of mine ever did. But they did eat the veggies in the gloop -which is a dish made out of cooked whole grains mixed with chopped or small-sized [like corn and peas, for example] frozen veggies. And greens - they LOVE greens!
See, the thing is that although cockatiels are classified as granivores, what they eat in the wild is grass seed which is relatively low in protein and fat not only because of the kind of seed they are but also because they eat them off the plant when the seeds are still green. So, when we free-feed [filling up a bowl and leaving it there all the time] a dry seed mix, we are, in reality, feeding them a breeding diet all year round because all dry seed mixes are higher in protein and fat than any green grass seed they might find in the wild and this makes them hormonal all year round [NOT good!] and destroys their livers and kidneys in the long run. I suspect that the 'big' seeds she prefers are sunflower or safflower seeds which are higher in fat and protein than the 'little' ones [different kinds of millet] and that's why she prefers them [Nature hardwired them to crave high protein because an abundant source is not found easily in nature so nature made it that, when they do find one, they eat and eat and eat -same as we do with potato chips
].
The best diet for your tiel is gloop or a similar dish accompanied by a different kind of leafy green every morning and, for dinner, a small amount [one measure tablespoon] of a budgie seed mix for the winter and a cockatiel mix [with just a few sunflower and safflower seeds] for the winter [but yours should have only budgie mix since now because you have been feeding the high protein/high fat mix for a long time and you need to clean her kidneys and liver].
I also suspect your bird is overly hormonal because, in my personal experience [I've had a flock of over 30 tiels], females only vocalize when hormonal and alone. Cockatiels are supposed to be 'easy' parrots to keep but, although they are easier than other parrots, you need to know what you are doing because they are VERY opportunistic breeders and if you give them the smallest chance [and I do mean the SMALLEST!] to start producing sexual hormones, they will so they need to be fed a low protein diet during the winter and kept at a SUPER STRICT solar schedule with full exposure to dawn and dusk [and make sure no light whatsoever reaches her cage once night falls or it won't work].