Hi, I'm Heather, an attorney with 15 years experience, and I'd like to assist for informational purposes.
I think that you should bring out the point that your boyfriend has assumed the father role to your two children, and that in doing so, he has stepped into the position of providing for your children as a parent provides for a child emotionally, mentally, and financially, and that a long prison term, will have a negative impact on your two children, who love him and rely on him as a father, and who deserve to have their emotional, mental, and financial needs met to the highest standard possible.
In addition, I think that rather than you stating that you think your boyfriend has learned from his crimes, maybe you should point out what steps he has taken to rehabilitate himself after committing the crimes - any classes, counseling, or something similar.
You could also touch on extenuating circumstances, which you understand do not excuse him of committing the crime, but reduce the crime's gravity from the point of view of punishment or moral standards. You talk of "obstacles" but you don't go into detail on any of it. Since you don't give any detail, you have prevented the judge from feeling any sympathy for what your boyfriend has gone through. For example, if your boyfriend was abused as a child, or if he both his parents were murdered, or some other traumatic event left him negatively affected, you might offer that as a reason the judge might give a bit lighter sentence. But in order to have the judge consider it, tell the judge what happened. If you don't give the judge the details, there's a chance no one else will either.
Also, your wording of - when you say "the crime as well as his prior offenses." - - you mention the prior offenses alot and each time you do that, it makes it seem like he is a re-occurring offender. That is not something you want to keep reminding the judge of, so I don't think you should continually mention "prior offenses." Instead just say his crime, or his crimes.
Finally, I think that if you are engaged, you might call yourself his fiance, rather than girlfriend. It makes your relationship appear more stable and serious. Especially when you write about the fact that he is the father-figure to your children.
Let me know if this has answered your question.