I hope anything I suggested helps you. I feel I rasied our 5 children a single parent. But he tried to be there when he could. He has made up for what he missed with the older 3 with being home for special things for the younger 2.
I learned that doing things for myself I became stronger. By becoming stronger I leaned less on him and liked the independent me. But it is a fine line to too much independence, if you become too independent you think you can make it with out him. And, you probably could, but you want him in your life. You have to reassure him that you want him. You do that at night in bed and when you plan special time for yourselves.
Do you make a date just for you and him and only do something that is fun to the two of you.
We used to go car shopping on Sun afternoons. We would just go to car dealers and look at new cars. We were not ready to buy but it was fun for us to look and maybe testdrive.
We just bought a pass to a local state park. We thought it would be fun to go there one night at sunset, take a blanket and sit by the lake maybe make out (lots of kissing, no sex ) until the park closes to noncampers. Inexpensive, but something we enjoy doing. We are going to take the kids camping one weekend when it is cooler to share that with them.
Look back and see what you did when you dated for fun, see if you can duplicate that.
I re-read your post - I think all of us wonder sometimes if a "third"person is in our bed. I remember nights making love asking later if he had done the same thing with her, that to me made it less somehow. He got upset and told me she was never as creative in bed as I was. That made me feel better, but he was disturbed that I would bring it up at that time when things were good. I couldn't explain it, but told him he would have to deal with it. I have gotten past all that and things are good. So you will get past that too if you allow yourself. You have to learn to let it go.
You might consider forgiving her and him. I don't say forget, just forgive. By have ill-feelings towards her you create negative feelings in your life that harm you. Remember that what goes around comes around and she will get what she deserves in the end, but it will not be from you. You have better things to do.
You are going to create the best marriage you and your husband can. You are going to raise your children to be beautiful people like you. (I do not mean physical beauty, but inner beauty)
You are going to find goals for yourself and be the best you that you can be.
Well enough from me. I have been long winded again.
Above all else, "To thine own self be true."
Texasgirl