First, I want to commend you on putting your children first. This is a very difficult situation for you to go through and keeping the "mask" on is difficult as well. For too many reasons, I won't go into now - I feel for you and the situation you're in. Unfortunately, you are not alone.
Moving forward, you seem to have a level way to looking at things and keeping yourself on the high road. As you stated, it is not in your nature to be under-handed, and you should stay true to yourself. Just because he hasn't honored you in this marriage doesn't mean that you shouldn't honor yourself!
I can certainly understand you wanting to wait until after your daughter's birthday to begin this process of separating and possible divorce. It is a selfless act and also will assist your daughter, who is at a critical age, not to associate her special day with this situation that will surely affect her.
It is also very smart of you to seek out options to garner & save more money. Hopefully, things will stay civil between you and your husband as this all unfolds, but it is not a given.
I think you should continue to be in cool, calm communication with him. If he has no idea that you are aware of all of his indiscretions, at some point the conversation will have to "go there". You certainly don't have to give up any of your sources or ways that you know about the details, but be prepared to let him know that this is the main source of the rift and reason for separation and that you can no longer tolerate living this way.
Stay close with your female friends and relatives - they will help you through this. Life will not always be this way. As a wise, female elder once told me, with regards to being in a similar situation - "you can't be unhappy forever". It's not about loving him, because I'm sure you do - we cannot turn our love off or on like light switches - however, we have to make sure that we don't have ourselves in situations that make us wear masks. You, just like him, have a right to enjoy your life and do what you wish...and with whom. Know that you are going to be parent partners for life for your children and do your best to rise about any pettiness and keep their best interests first.
I truly wish you well. Stay prayerful and positive.