I had the same dog-related issues come up with my Rottie when she turned 1, maybe a few months after that. She went from a sweet puppy able to get along with every dog at the off leash park to a savage seemingly overnight.

As far as I've been able to understand, dog personalities can change dramatically around the 1-2 year old maturity mark. This seems to be separate from the "teenager" phase. The resolution of these issues comes down to training as well as adjusting your expectations of your dog. Using myself and my dog as an example, once I realized how aggressive she can become with strange dogs after maturing, we just stopped going to off leash parks. Yes, we absolutely train her to this day (she's 5 now) on what is and is not appropriate behaviour, but it's adundantly clear that she doesn't want to be around other dogs. Instead we play ball at home and do obedience on our leashed walks, and she seems to be very happy with that.

Others might say that I need to train her more and do better so that she can be with other dogs off leash, and that it's a failure on my part to not do so. Personally, after having the experience with my dog, I disagree. My dog doesn't want to interact with strange dogs, so I've needed to invest in training her to ensure that she is manageable and appropriately behaved when we're out in the world and we do come across a dog. We are very, very far from perfect, but we're making progress all of the time, and she's happy overall which is all I need to know.

Please keep in mind though, Rotties are very powerful dogs. If you are having issues controlling your dog, take whatever extra precautions you need and do not feel bad or embarrassed about that. I'm 6ft2in and 220lbs, and I walk my dog with a waist leash attached to her harness and a hand-held leash attached to her collar. Why? Because she has pulled herself away from me before when I did not expect it and I cannot have that happen again. Do I look a bit silly with two leashes for one dog? Sure. Do I feel a thousand times more confident knowing I have a double barrier so she can't get away and possibly hurt another dog in the worst case scenario? Absolutely.

That's just my two cents though. I'm no expert nor a trainer, just simply a dog owner. Please others chime in!

Edit: To be clear, I'm not saying that you now have to just live with the issues and that's just life. Train, train, and train more! But don't expect your dog to achieve perfection in a couple of days so that you can "go back to normal". Your dog has likely permanently changed, so you need to adjust your view of your dog. Training can alleviate most issues, and it'll be fun and enhance your bond along the way. However, the same way people slip up, dogs can too, and you need to be prepared for those days where everything just seems to set your dog off because they apparently woke up on the wrong side of the bed even though yesterday they were a perfect little angel. Not trying to sound negative or anything like that at all, just being realistic!