Should I let my husband have a gf?

So here it is: the marriage with my husband has been ROCKY for a while. With 3 small children (back-to-back pregnancies), he has asked for more intimacy (for years), and I have been unable to meet his sexual needs. He has asked repeatedly for a girlfriend so that she may meet his sexual needs since I am unable to meet his.

Other details: He has also expressed that I don't value or respect him. He states I don't listen to him, among many other concerns. We have gone to couples therapy, I have been in therapy, he has gone to therapy, I was diagnosed with prenatal and postpartum depression for all 3 pregnancies, and within the last month, I was diagnosed with ADHD. I don't know how to fix my behaviors to make him feel valued and respected, and I'm at the end of the line.

My husband and I have been married for 8 years. We have 3 small children; we have always both worked full-time. And The current role that I have is stressful, especially these last few months.

Also, my libido has consistently been low since my first pregnancy, and this has been a constant issue for almost 7 out of the 8 years we have been married.

To say we have had high and low points in our marriage would be an understatement, and I officially feel out of gas to improve our marriage. And I am at a point where I believe if I don't say yes, to my husband having a girlfriend, our marriage won't survive. Tonight, when we calmed down from our argument, he stated that our problems would be solved if he had a girlfriend. And I, in tears, agreed. He was surprised and he stated that a girlfriend would drive me crazy and i admitted it would. I told him that him having a girlfriend would be painful but losing him would be more painful. He then said, I do not understand why I can't just fix myself. And I said if I knew what was wrong with me, i would have already fixed myself. But I want him to be happy, and he has needs, and I know that I am currently not doing that for him.

I am so embarrassed to post this here, but I am more embarrassed to tell anyone this, even my best friend. Can anyone please share their experiences on this issue in their relationships? I am obviously not posting everything here and I am not perfect but I do love my family more than myself and want to save it. Thank you for your words, please be kind, I know the idea of letting my husband have a girlfriend is already crazy.