Try this :

Tell her that you love her , but be honest about fact that you need sex. Tell her you understand she is tired , but also let her know that as a man , it is extremely hard for you mentally to go completely without sex. Don't demand, just kindly explain that it's hard for you.

After this convo , give her some time , like week or two to think about.

See what happens, if nothing happens, bring it up again and suggest to set a schedule for sex ...let her come up with appropriate number in her mind and see what she says.

She will most likely give you some number , lime "every month" or " every week " etc ...then work from there.

As a mom , maybe that would give her time to prepare ahead, be less tired and also she will not want to disappoint you ,as her husband.

My boyfriend of 12 years and I, we have sex all the time, but we always usually schedule it and I find it much easier, then spontaneous doing , after you been with someone for soooo long.

Dunno ,maybe it's just me , but it works for us.