I am a middle child of 3 girls, and absolutely have middle child syndrome. IMO it’s worse when the three children are all the same gender, as with mixed genders you can still be the ‘oldest girl’ if you are in the middle.
I just never felt loved enough. My parents all love us so much and have supported us financially and emotionally and in every way possible, but I just knew that I was third. That if the others wanted to do something (particular day out, particular meal for dinner, movie to watch out or on DVD - whatever we were picking) I wouldn’t get my choice. They might make a token effort to consult, but I always knew it would not end up as mine. Like others above, I guess after awhile I just accepted it and didn’t bother to speak up, but the resentment was still there.
Like others, I notice it impacts me at work. I will see something wrong and won’t speak up as I will think I can’t possibly be the only one to see it, or the only right one, and then it becomes a big issue and I kick myself for not backing myself, and speaking out and showing my worth to the company.
My parents deny it, they say they loved us all equally, and I believe they think that. It’s not about the loving, but I think about an easy life. If the eldest and youngest are the loudest, then you cater to them to keep the peace, and your middle one disappears.
What would I have wanted? More equity in the decisions. I guess like, Eldest gets to chose the family movie tonight, but it is middles turn next time (and stick to it). One on one time with each parent - days out etc. My siblings got one-on-one time with parents - the eldest before I was born and the youngest in the gap between me leaving for uni and them leaving. Both very different times, but my parents speak of both those periods fondly but have never realised that they don’t have a similar time for me. I was always in a crowd.