It sounds as though this is a really painful time for you - I hope it gets better sooner then later.
You say that J is maxed out with intimacy with you, I'm reading that as a need to explore himself and others apart from you. It also sounds like you have talked with him in detail about this (is that the case?) and he probably isn't going to suddenly provide you with more 1-on-1 time anytime soon. I don't think asking for more of his time is a good idea - it may appear needy to him and thats probably not the feeling you want to promote (not that you might be contemplating this, but just in case).
You acknowledge a level of personal insecurity, and the fear of losing the relationship during confrontations with J. How severe are these feelings? Sometimes when couples argue or disagree, the relationship can actually improve because people can air their grievances with one another. But rejection-anxiety issues can be a huge block to this process - for both parties. He may feel as though he can't ever be completely honest with you for fear of triggering your anxiety. This can be very taxing on your partner (and you), and may be part of the reason why he wants more time away from you.
What steps have you taken so far to understand the basis for your anxieties? Are there people other then J that you can talk to about this? Maybe finding ways to reduce the triggers for your fears can make it easier on both you guys. Anxiety, in general, is a debilitating situation to be in. Finding some way to reduce those feelings might go a long way for you two.
Take the risk of thinking for yourself, much more happiness, truth, beauty, and wisdom will come to you that way. - C. Hitchens
Me: Male, bi, 48, flexible