I second the amount of energy being something you can't actually grasp until you are living it. Some Boxers are lower energy than others which means that *some* are beyond anything you can imagine. Both of my girls have been off the charts as far as energy levels are concerned and I have been lucky in that my work schedule has always allowed me to provide (what I thought) was plenty of mental stimulation and vast amounts of exercise. Sadly mistaken in that arena!!
Don't be alarmed or ashamed of "puppy regret". It happens.....I spent weeks crying every day when I brought Cami home nearly 9 years ago. She was just SO MUCH to handle. Thinking I knew what I was getting myself into with Raine and there wouldn't be any issues I couldn't handle I had blinders on. This girl, now almost 20 months old has thrown me for a massive loop. She knows every single one of my buttons and pushes them daily.
Learn to deal with a stubborn dog who thinks they know it all. They are quite intelligent even if they don't always act like it and the two make an explosive combination. I like to say that I can make Raine do anything SHE wants to do and it really does fit the bill.
You will read that Boxers are a sensitive breed and this is 100% true but it also means so much more. This sensitivity affects them in so many ways and you will come to realize over time what your new addition can handle and what they can't. My DH and I know that we can't have a conversation through a door (him on one side and me on the other) and use loud voices to be heard. Both of my girls have reacted negatively in these situations. I don't know if the louder voices is upsetting (I assume so) or if they don't understand why they can hear two voices yet only see one person. They are happiest when their whole family is happy. They can sense your down days with sadness, bad moods and times of joy. They feed off of human emotions. Learn to keep yours in check so they are not affected. A sad or worried Boxer is a troubling sight!!
If they want something they will figure out a way to get it. If this means climbing on something they will. If they need to dig to China to find it they will. If they (when they) figure out that whining gets them what they want they will do that too. They are problem solvers plain and simple. If they can't by any means figure something out they will use their humans which in reality means they DID go about figuring out something they just added a person into the equation.
Boxer learn differently than a good portion of other breeds. Most dogs learn by repetition. Boxers have a short attention span (if they have one at all) and they get bored very quickly. As much as their bodies are in motion....so are their brains. You can literally watch a Boxer and know when they are thinking. I received this info from my trainer friend.....
Teach a Boxer to sit by showing them what you want. Use the word with the action. They learn (as most dogs will). Continue to ask a Boxer to sit and you get something like this....
Sit.
Boxer Sits.
Sit.
Boxer thinks about what else might be more fun.
Sit.
Boxer has thought that checking out something in the corner will be amusing to them.
Sit....
Boxer is no where to be found!
Change things up and you will get far better results much faster! Don't think your new pup is too young to start learning. Don't overwhelm them but do start them on a routine the day you bring them home. Don't allow things that might be cute NOW as a tiny baby that won't be so adorable as a 60lb teenager (who can be an idiot) or an adult.
Reading as much as you can from this forum will provide some insight and a lot of useful information but until you are in the trenches you won't really know what to expect. To love a Boxer and to be loved by one is one of life's greatest joys. Considered yourself owned by a Boxer and enjoy every minute of it. I have yet to meet another Boxer lover whose life wasn't changed for the better by sharing their life with one.
It is truly an amazing adventure!