No trust both ways

I have been in a relationship with my husband for 8 years, married for 3. It has been rocky over the years as there are a lot of things in the past and present. I have one son with a previous partner, he has 4 children with 3 previous partners (including an ex-wife). I am going a bit insane at the moment because things have started to get ugly again over contact with others/lack of trust. I cheated on my husband (with my son's father) in the few days after we got together and before he moved in (as he was leaving his ex). I didn't tell him about this until later on in the relationship but he still stayed with me and proposed etc etc. Over the years I have liked attention from others but have never acted on anything, there have been rumours about him doing things with one of his ex's and even his brother's girlfriend but this was apparently contrived by angry ex's and interfering so-called friends. I want to believe that he hasn't done anything but I do not trust anybody. What has started it off again is that he has recently gotten friendly with his son's mother again and is imessaging her a lot. I hate to admit it but I have been looking at his phone behind his back and have found messages talking about her relationship with her husband, her weight and how she is doing, even making reference to her 'titties' and jogging. They often/mostly put kisses on the end of their messages and there was 'Huni x' used at one point. The ex said about her problems to which my husband responded 'you can always talk to me', ex said 'don't think ------ would be happy about that', HB 'why say that', ex 'you texting your ex all the time', HB 'it fine'. I have voiced my concern that he is always texting her (his phone barely leaves his side) and that if it were me texting my ex he would go mad - this caused an argument and he brought up all the things in the past - me cheating on him with my son's dad, also saying that he thinks there have been others (either in the beginning or along the way). Nothing I say to him can convince him I have not been unfaithful since that one time back in the beginning and I cannot believe that he hasn't done something/won't do something. If I try to bring anything up he gets defensive, throws it back in my face and it then all turns around to me and what I did, what I am like, how I have got no leg to stand on etc. He doesn't actually see why I am getting funny about the contact. Or am I being a paranoid idiot??? I love him dearly and want to spend the rest of my life with him (hence why I married him) but there is always this dark cloud hanging in the depths of our relationship and I cannot resist the need to check his phone etc. I have had to shorten the details a bit as I don't want you to fall asleep but any advice on what I have given would really be appreciated.

You both played the game and while you have stopped, your husband has, going by your post, continued and hence your lack of trust. Now you have conflict and little or no constructive communication. Frankly, regardless that you want to spend your life with him, you both really have no business being together if you don't have the foundation of all relationships...trust. You have chosen this man but in his mind, he is justified to do what he likes but the one thing he's not doing is respecting you or your relationship together. No partner has the right to dictate terms in a relationship but if your relationship was smooth sailing, then he most certainly wouldn't have the attitude that he has. It's all very well to text an ex but there are boundaries which need to be respected by both parties. The previous poster is correct, you need to set the standard but you also need to commit to somebody who shares your values as well. Your children together need a happy and stable environment to prosper but while, as you state, you have a dark cloud hanging over your relationship this will be unlikely to happen.