To the OP-
Sins are either material or formal-that is, they are either unwitting participation in something that is itself a sinful act, or they are wilfull participation in a sinful act.
Before I go on, just want to clarify that I am not saying that eating meat on fridays during lent is not a sin-it is, but not because the actual act of eating meat is sinful, because it isn’t. Eating meat is not in itself evil regardless of circumstance, as compared to something like abortion, which is always an evil thing. Incidentally, you can eat cheese on fridays during Lent-eggs and milk, too. Just not actual meat.
The actual sin at work in this issue, as I see it, is in one’s attitude toward the teaching authority of the Church, which I think is much, much bigger than just the act of eating meat, and which is also why eating meat on fridays during Lent is taught to be a mortal sin.
As Catholics, we have a responsibility to uphold the Church’s teaching-when we don’t, willfully or not, it can create scandal and can negatively affect the habits of faith and discipline that we should all strive for.
You did not formally reject the Church’s teaching on this issue-which would be a formal (and also mortal) sin.
However, you did materially fail to uphold the teaching of the Church, not because you disregarded it (which would be formal), but because you forgot, which isn’t formal or mortal because of the lack of intention and the fact that mortal sins really can’t be accidental.
That’s as far as I can go with this, so I guess I’m saying that yes, you made an accidental mistake-we all do, and we all keep trying to do better.