also fyi to answer the question more directly, i grew my hair out with sew ins. back to back for two years my hair was never exposed to manipulation or heat damage so i retained every inch of it. if my hair grew 6 inches a year, it would have went from SL to BSL in those two years. it grew back to apl, then took another 2 years to get to where it is now. people are saying they're stuck at SL because their hair grows slowly and they have setbacks/cuts that lead to retention issues, well this is how i manage my retention, and my hair isn't crazy longer than theirs. just by watching my hair as it grows it's obvious it isn't growing six inches a year.
Could it have been that the majority of your SL hair was already damaged and caused it to break off? No one seems to have a problem getting from a TWA to SL in a year. That's 6 inches.