My husband only has sex with me once a month or less

This is often a tough conversation, and most try to avoid it. It's understandable, and yet, it's a good conversation to have. How else would you know what's going on? I would probably start not by expressing your frustration, but by asking him what's going on for him.  Is he happy with once a month situation? Would he like to have more intimacy, sexual and non-sexual? If he does, what's in the way? Medical issues, stress, confidence, performance pressure, sometimes people are just too busy and tired ... The question then becomes how do you two can find more time, what would feel good for both of you? 

If he doesn't want to have more intimacy and once a month works for him - then it's a different conversation. Desire discrepancy is very common . Your needs, wants and desires are different. You two need to figure out what works for both of you, as a couple. Understanding each other is key. 

You are feeling frustrated already - understandable. Are you able to share with him what is it that you want or need? it's common for couples to experience a decline in intimacy. We get too comfortable.  It's too familiar. We lose the spark.  It takes work to bring it back into the bedroom. It takes effort and commitment from both people. We often simply don't know how to do it. I think it's starts with a conversation, then maybe a research project, then a lot of experimentation - learning about yourself, each other, what works and what doesn't. Good luck, and don't give up please, it's possible to turn it around.