Hi, I'd like to help you with your question.
It sounds like you are making a serious effort to make your blended family work. You've tried many things that are recommended to help work the problems out with your marriage and your family.
From what you said, it sounds like you inherited a family when you married your husband, made an effort to make it work, and are now finding that you are still an outsider. It makes it difficult to be part of the family if you are not included.
You are saying that you feel you cannot put more effort into the marriage because your husband is happier with his kids and you are happier when you are on your own without the kids. It sounds like your husband has not helped to put you first as the other parent. It seems he is putting the needs of the kids and his bond with them over his relationship with you. If he is not willing to put you first, then it will be very tough to make your marriage work. It is partly his responsibility to initiate the relationships between you and his kids and you and his relatives. By not showing you are first in his life, he is letting others push you aside as well.
Even in marriages with both biological parents, the husband and wife have to function as a unit. They cannot put the needs of the kids (or anyone or anything) first or the marriage will not survive. It's like on the airplanes when they tell you to put your oxygen mask on first before your children, you must make sure your marriage is strong (and breathing) before you can make sure the children are ok.
You have a couple of options. You can talk with your husband again and mention what we said here. See if he is willing to work on this angle of your marriage. If not, then you have another choice. You can try a trial separation. It will give you a chance to see if things improve for your marriage and for you.
You can also work on educating yourself about your situation and see if you can try anything else before considering ending your marriage. Here are some resources for you:
Should I Stay Or Go? : How Controlled Separation (CS) Can Save Your Marriage by Lee Raffel
How to Know If It's Time to Go: A 10-Step Reality Test for Your Marriage by Dr. Lawrence Birnbach and Dr. Beverly Hyman
Remarried with Children: Ten Secrets for Successfully Blending and Extending Your Family by Barbara LeBey
You can find these on Amazon.com or your local library may have them for you.
I hope this helps you,