First there is no circumstance under which wanting to serve your country is bad. Yet, you are really not doing it for that reason. You want the United States Military to solve your problems rather than you being the greatest asset you could be to theirs.
Discipline is something that comes from within. You have to make yourself responsible for your actions and trying to rely on others for this is sadly a recipe for failure.
I don't want you to fail. I want you to succeed absolutely positively because at 28 years of age, you have a great deal of life yet to live. You may be the one who brings something great to this world and make an incredible difference and improvement.
My suggestion is to begin a program of self improvement and do it in stages. The first stage should be saving a few bucks and researching the supplements you will need to accomplish your physical goals or transformation. If you are one who is subject to a great deal of physical discomfort or muscle soreness after physical activity, make certain that you have supplements of BCAA and Glutamine which will minimize this post workout pain. (The pain comes from the buildup of lactic acid) It is this post workout pain that often turns people off to physical exercise.
Look my young friend, I have 30 years on you and a great deal of life experience. We have a young kid just out of high school in our area that has just enlisted in the Air Force. He is basically a good kid, but, his physical condition is such that the odds of him making through his basic are not good.
He is over 6'4" tall and is still under 150 pounds. The lack of muscle mass is astounding and this kid is just a human stick. He was given information by his recruiter that with relation to physical testing is pure manure. If he gets through the physical parts, it will be a miracle. The next part with this kid is that he is mentally not what I would call at all mature. He is by a calendar 19-years-old, but by his behavior he is still about 15. He had a sad story in that his Dad died of cancer at a very young age about a year ago. The last formative years he had with his family revolved around his Dad's cancer therapy and this kind of took away from what he needed at his stage of life. This young guy by nature is an individualist and not one who is excellent at "team thinking", he is not one who is well disciplined in his own life, his H.S. grades at graduation were to be honest less than stellar and it was because of his own attitude that they were so low. In my day, there was an old rule. "Put the kid in the Service because it will make a man out of them." I left HS in a graduating class of over 700. Many of those did not come back a man, it was the Viet Nam War and many came back in body bags. Others came back with severe drug and or alcohol problems because they did not have the mental discipline to be able to function in a combat environment.
The United States Military IS with little doubt a great thing and for some people that function will under the necessary regimentation and military mentality. It is not for everyone and based on your reasons for wanting to enlist it does not sound or promising for you. It comes under the heading of the right thing for the wrong reason.
Whatever you choose I wish you the best, but I would work on self-improvement in the best way and that is because you yourself have decided that it is worth working for.
