I am in no way an expert, as most parents likely feel obligated to claim before proposing an answer to another parent's "problem". We are all really just a bunch of kids who grew and had kids of our own.
Having said that, my second daughter also had this issue, and honestly to this day has issues with falling asleep at night. She is now (I believe) 4 years old. The key here is to understand why your child has such a difficult time falling asleep, and trying to deal with the root accordingly and not the symptom itself.
In my research at the time, it turned out like any issue really, there are a number of possibilities. The most prominent of course being over-stimulation, especially in young children, and even more specifically in young children who may have hyper active tendencies (thats my nice way of avoiding the psychology catch-phrase "ADHD", because honestly we all know its a bit over used).
Since your child is still so young and therefore incapable of understanding or even communicating their feelings, you will have to go through the process of elimination. Start with a bed time routine if you have not already. The most critical step here being to identify when your child naturally starts to get sleepy, because forcing a sleep routine on a child at a time when their activity is still at its peak only leads to stress for both you and the child. Do away with any stimulating events, activities, surroundings, etc a few hours before their "bed time".
I dont want to detail any sort of process, for two main reasons: the first being that one size never fits all, ever. And the second being who the hell am i to tell you how to deal with your child?
Having said all that, I feel like i would not be helping if i did not point out one apparent issue with how you are approaching the problem (based on how you worded your question): this is more difficult on your child than it is on you. Your child is not "reverting" and nothing is "wrong". Some children, like some adults, just have trouble sleeping. You as their parent are really there to help them understand how to deal with a problem that affects them personally at a time in their development where they are incapable of understanding the issue well enough to address it themselves. NOTICE: I am not calling you out, or saying you are a bad parent. I am simply telling you what I wish more parents would have told me: your kid is going through this, and you are sitting on the outside observing and HOPING to help. Never lose sight of that, stay patient, and remember some problems aren't really problems, but rather facts of life.
Wishing your little person the best of luck, as I know first hand how difficult this is for a child. Not because of my daughter, but because I myself suffered through sleep issues from childhood up until this very day.
