Lets put this simply :

1. If you WANT to fight for your marriage - do NOT DATE OTHER MEN.

I KNOW some people will tell you H may get jealous. Yes, he very likely will. How will he respond to this jealousy?

a. He could end his affair with OW and ask you to make a committment to him
b. He could pursue his affair with vindictiveness and a clear conscience

Which is the most likley outcome... A or B?

Now, I see a lot of hopping around. You have indicated you are confused etc. This is NOT sending a clear message to your H. I know you think he knows how you feel, but trust me. A quick scan of your post tells me he's getting way too many contradictory messages.

1. You tell him his affair is hurting you and you want him to back off of her... And YOU accept a birthday present from her? To a GUY that means you changed your MIND about her...

2. You work with her voluntarilly? BIG STOP SIGN there. If you can do ANYTHING to show you do NOT support his affair.. you DO IT. This means exposing the affair in the workplace and refusing to work with her. Or get her fired if you can do that. Otherwise work from home or find a new job. Do NOT TOLERATE HER. When you show tolerance for HER, you show your HUSBAND tolerance for his AFFAIR.

let me repeat that :

Do NOT TOLERATE HER.

Show tolerance for HER = show tolerance for H's AFFAIR.

if you want to END an affair you must NOT TOLERATE it. Being FRIENDS with HIM and HER and acting nice gives him a free ticket to continue... you are basically telling him you are ok with an open marriage. You have a child right? You need to start thinking like a mom and PROTECT YOUR HOME from threats. HE is not being a father or husband right now. He is allowing threats to his home dance all over the place and YOU are SIGNING OFF ON IT.

YOU need to commit to your marriage and a full scale attack on this woman or DIVORCE and move on. You can't fight an affair halfway like you are doing... it just keeps going on for years and leaves you getting anxiety, depression, and a lot of other unpleasant emotional and physical illnesses...

If you commit to your marriage
1. Expose the affair - I don't care if it's EA or PA. Expose it and find friends who support you on getting this woman out of your life.
2. Tell your H on NO UNCERTAIN TERMS will you engage in convo with HIM OR the OW out of respect for your daughter and your marriage. It's time to take your dignity and confidence back.
3. Follow through on 2. Do NOT engage either of them. This OW is going to try to smooth things out and be nice or whatnot - You need to steel yourself from that and SHUT HER OUT... along with H.
4. Get to work on getting yourself and your child supported independent of H. SHOW him you mean BUSINESS.

He's going to deny any affair is going on.

5. Print up articles from the internet about emotional infidelity. Glass has lots of great stuff to say about keeping emotional walls up to threats to your marriage - you AND your H should be doing this... He needs educated.

Print them up... give them out to anyone who will support you. You are starting a campaign to shame this woman out of your life and your H back to reality.

6. Take care of yourself. Do NOT date. It's ok if you LOOK attractive and men invite you to dinner, but do NOT go with them. If you want a monogamous commitment from your H you need to SET a GOOD EXAMPLE of that... dating other men is NOT setting a good example of a mother and wife committed to her marriage... Your H must follow YOUR LEAD...

I can add more later, but right now the halfway db commitment needed to be said like a house on fire...