I don't have a personal experience, and how could I have one since I don't have a middle name! But logic tells me that you won't have any problem: perhaps if your middle name were Sebastian and there they had written Sebasti somebody really fussy at check-in would mind but if, as you say, the middle name in your passport is indicated as Mynameis24letterslongyes, absolutely no agent would mind if the reservation has Mynameis24letterslongy instead.

Now, why don't you try to use an initial in the future? I would make both your life and that of check-in agents much simpler.