Huge relationship hurdles...help please

Before you judge me or any other person involved just read through first. 2012 I met my now fiance, he introduced me to a fair few of his family and friends very early on and I became best friends with one of his friends. I was pregnant with our first child and my now fiance cheated on me with his ex. I split the relationship instantly and we had no contact for around 6 months and at this time I had given birth. We made amends and tried the relationship again and I fell pregnant again with our second. Thing a were going great, however, I had a fair bit of mistrust with him but wee worked closely together to make things right. However, things got sour and he became verbally and mentally abusive. But I stayed thinking I deserved it, because I had so little trust in what he was doing. We went on to have a break up and came back together again and I fell pregnant with our daughter after two miscarriages and all was amazing for 15 months and it went sour again. We Had our daughter, lost our home, lost our car, I was at my wits ends till we found a home in another state. We did a new start for our family and it was great we were solid and happy, we got engaged and things went sound in April last year. As much as I've wanted out I couldn't bring myself to leave. He accuses me of cheating, having a boyfriend, having sexual relations with the neighbours and it put me in a suicidal depression. However I still love him and I don't want to hurt him but I know I need out. The problem is I know what I want and need but I can't physically break it off, I would do anything for him but I am feeling a certain type of way about his friend/my best friend and he feels the same. He came to me and admitted how he felt and I very much just brushed it off and said okay, thanks for be honest but you know I'm engaged and so on...deep down I wanted to scream that I felt the same way. Now I don't know what to do, I want to tell the best friend how I feel and he has held it in for 8 years and I've held it in for about 5 years, I know the life my kids and I would live outside this abusive relationship would be perfect. So I'm really falling big for the best friend, I always have wanted to tell him how I feel and now the opportunity has presented I need to say something but I also don't want to hurt my fiance even though he lies to me, cheated multiple times, verbally and mentally abusive, he screams at our kids for no reason. I know he has a really sweet side to him, I just don't know where it has gone or if it will ever return. As soon as we have an argument he tells me we are over and that he feels like sleeping with a prostitute. But he always comes back with the I'm so sorry, I shouldn't have said anything like that, you're amazing and all that jazz. He gets very protective of the kids and I when things are bad and can be really loving and kind hearted when he feels like he needs to be. Leaving him is not my major problem, my major problem is how would I Persue a relationship with the best friend when it is his friend too. I don't know what to do. I don't know how to feel. I just wish I had some direction 😞😞 However

Hey, Ok let's start with the very beginning. I'm not sure if this will make you feel better but the majority of people that are in an abusive relationship don't know that they are in it, or realize it, when it's to late. When you decide to have a child you need to think first, if you have the conditions to have a child, things like a stable relationship, a stable job, house and one of the most important things would be a calm and great environment at home. Even small children realize what tension is, and that stress can be extremely violent for their growth. And I am very concerned by it. Whatever you do, or you think on doing must be for their greater good. And if the man you now like realizes that to, and it's able to provide for their care, then you shoot two ducks with the same gun. You and your kids need to get out asap. You are thinking things ahead of the train. When the time comes for you and him to be together and if you really like each other then it will happen, nothing else will matter after. Because you don't own your soon to be ex an explanation. What makes you think you do? He didn't gived you any, he doesn't deserve the family he has. Anyone can be the greatest people of the world on the inside, but you own the people you live with to show it. If you don't respect the people that you are with, if you say that making love with your wife is like sleeping with a prostitute and verbally abuse her, then whatever you are expecting him to do, don't. Think about your kids first and then decide how you are going to do it. If the house is yours talk to him about divorcing and make him leave. If the house is his found a new place.