A few pieces of advice.
- Insist that M take his relationship with you seriously, not disrespect you, and be 100% honest. Tell him that if he can't do that it will badly impact your marriage, plain and simple, just a it would if anything else in his life were causing him to treat you poorly. Make it clear to him that none of those things involve not seeing N, maybe show him this thread to try to help him understand that. Promise him that if he lives up to the commitment to do those three things you will not guilt him or pressure him or in any way give him grief about his relationship with N.
- To help M keep the commitment to treat you right, ask him to read up on NRE, because that is in fact what's going on. Maybe even ask to sit down with M and N together, the three of you, so you can explain what actions have hurt you and why, and can make it clear that you support their relationship just not those inconsiderate behaviors.
- Treat your relationship with H as its own entity. If you're not getting what you want/need from it, then there's no reason this has to be a quad rather than a vee. You could go out on dates, maybe meet a new poly or poly-friendly guy who will give you more time/energy.
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.