There are 3 paths you can take,
1) Either confront him, leave if you are not vested enough. You will get over him... eventually
2) Accept your husband is polygamous, tell him that but you demand equal time and resources from him. All on the open. It's gotta be announced to the family, friends and all.
3) You keep on baring it all inside you and let it eat you up.... this option I highly do not recommend.
all the best... choose one and follow through with it.
There is an Option 4... which involves forgiving him, if you can... and hoping he won't do it again. Whilst giving him what he was getting from 3rd Party.