Urgent - Stepson relationship with me causing rift between our marriage

After reading many posts from multiple people, I've come to realize my story is very similar to their. Since every person's story is unique in its own way, I wanted to get some feedback on what I should do. About four years ago I got in a relationship with my wife. She was just in the process of finalizing her divorce and with her she brought two children of her own. At the time, the boys were 18 months old and 12 years old. My 12 year old stepson was not born from that marriage, he was born from when she was in high school and his dad was not a part of his life after he was 4 years old. Being pregnant caused her to have to drop out of school so she never graduated. As with any single parent, she had to work multiple jobs just to make ends meet which caused him to not really have a lot for a while. After different relationships and different men leaving his life, eventually she got married. Her marriage was not the best and in fact led to it being abusive to not only her but to him. This was the main reason why she got divorced. Around this time, is when I came into the picture; I was 24 and she was 31. All my life I've always been more mature than most so the age difference was not a problem for me. I knew that it was a problem for her because she was worried I was too young. Nonetheless, we pursued our relationship. The first year was fine. We got along because he was still a kid and we interacted in different ways. About a year later, my wife and I got pregnant with our son. The stress of having a newborn and moving to a new place caused a big drift between everyone. I did forget to mention that even at 12; he still needed to be reminded of his chores constantly and would not listen most of the times to instructions given to him. Okay, let me keep going. To prevent a lot of conflict, I took somewhat of a backseat to the issue and tried to not bother him as much because I thought it was a teenager thing. My wife always wanted me to be more of a authority figure in his eyes, but I knew well that this would never happen. He doesn't really respect anyone (it shows in his school grades and behavior) and any bit of authority I tried to show got responded with hostility. Even though I moved the family to a much nicer area, he still found the bad crowds and is constantly smoking MJ and staying in his room full time playing Xbox. My wife and I separated earlier in the year due to some issues we had but are currently living in the same household. We have decided to date each other and things were going well. Right now in terms of my SS, because of the fact that he does not respect me, sees me as a "man" based on his views, and doesn't see us having a relationship anytime in the future, my wife is questioning if it is best for us to just get divorced. My wife is siding with him because she had a similar rough upbringing so I don't have her backing me up. My SS is ready to go to the Marines and has researched everything about early enlisting. My SS has made it as though the reason he acts how he acts is because of me. He doesn't want to do anything to help because it would give me pleasure. My wife is okay with him enlisting but doesn't want him to go if its because of me. I understand that he is her first son and parents need to make decisions that benefit their children, but I don't know if it is fair given that we have three kids in total. Her other SS which is 5 now, we have a great relationship, and he loves me. The two younger boys respect me and love me, but our teenager does not. I don't feel this is all my fault and I do not want our marriage to end because of this. I need some advice people. Thank you.

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