D.K.
It sounds like you're both feeling protective and jealous on behalf of one of the children respectively. Step-parenting isn't for the faint of heart. It doesn't sound like she's got the makings of a step-parent. You'd probably be better off separated. I agree with the previous poster very much. I would just like to add that as much as it makes sense to spend your limited time with your older son, don't neglect the baby. You may think he won't remember anything anyways, but he'll remember your loving presence. He needs to know you and feel comfortable and close to you. That will translate through the years. Once that opportunity is lost, it's much more difficult to get that back. Don't let it slip through your fingers. That being said, I think a 2/3 and 1/3 split of your time should suffice. Your older son is at the age where he starts watching everything you do as he needs to categorize and identify with his gender. You truly do need to be providing guidance at this age. The baby has a limited attention span at this age, so a little goes a long way if you're not the primary care giver.
Bottom line, they both deserve your best when you have time for them. Common sense doesn't seem to work out well when dealing with young children. Love is the key, and you have to be there to give them that.
Maybe you could go to work full time and put school on hold until they are a little older. I would say, until the youngest is in pre-school.
If you're going to stay together, your younger son's mom needs to get her act together. You need some couples counseling and she needs some one on one counseling to help her cope with step-parenting. She should also think about finding a set discipline technique and parenting philosopy to follow, so she's not constantly trying to make it up as she goes. She will be a more balanced parent if she does. Some people don't need this, some people do. Maybe she could read some books since she has so much time on her hands. I recommend starting with "1-2-3 Magic" and "How to Talk So Kids Will Listen and Listen So Kids Will Talk" and even "Attachment Parenting." She should also do some reading about childhood development and what is reasonable to expect at which ages. Try to get her impassioned about child development. It couldn't hurt and I'm certain it'll actually help. Parenting is the most important job out there. We should study and train for it, just like for any other job. Maybe you could lead the way for her. I know you're already busy, but for your children's sakes you're going to have to find the time. Even if it means staying up later to study and carving out that time during the day to be with your children and learn about them.
Best of luck to you, whatever you decide.
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WOW YOU TWO!!! You all need separate therapists STAT!!! You should NOT be together, period!!! This isn't a healthy situation for these little people to be raised around. You both need to GROW THE F**K UP!!! You need to suck it up and start behaving like ADULTS!!!! For cryin' out loud!! This is NOT the right forum for this!! This is for advice. Quit trying to use US to settle YOUR FIGHTS for you. Split up NOW and go your separate ways! Mom, you get the baby, Dad you get the potty mouth 4yo. There, settled. Each of you has one child, that's fair. Now get AWAY from each other!!!