NEED ADVISE!

5 years ago me and my now husband got together. He grew up really close with another family due to their parents growing up together. The other family has 4 kids and he ended up having dating one of their girls for a little while around age 15 they did sleep together twice and she was his first love. She did him really bad always on and off, making excuses why she couldn’t see him but then she’d be at parties and cheated on him. After they split ways she always tried to wedge herself in his relationship with other girls. He still went around her family at any fathers because he thinks of them as family but absolutely had nothing to do with her for years. So when we got together he still hardly had anything to do with her. We got engaged a year later. She started sending old pictures of him to her sister saying she missed him and missed what they had. Well the sister told me about and my now husband and I had a sit down talk with the ex and he told her she needed to stop. That he found the person he wanted to spend the rest of his life with and what they had had been over and wasn’t nothing there for her to try and hold on too. She got real emotional saying she didn’t mean to cause problems and we took other the wrong way. After we left she started messaging saying we tried to tag team her and it wasn’t right. Time went on and my husband had other family that had done him wrong that he made amends with afterwards. He asked if we could forgive the ex because he wanted to put e writhing behind us as since that conversation things were good and she found someone she really loves. I wasn’t ready but he forgave her and they became friends again. They would check on each other every few months nothing out of the ordinary and no ill intent. The ex and I had a conversation months after they sorted their stuff out and she apologized to me for what she did and she that she was in a bad headspace and she never really wanted anything with my husband but one of those that could have been me situations but that’s just not how things were supposed to to work out. I forgave her and we wanted to start over with a clean slate. Here I am 3 years later and it is still bothering me. It was as bad when they asked how each was doing occasionally but she got Snapchat 2 months ago and my insecurities are out of the roof. I have spoken with my husband and let him know that it is bothering me that they Snapchat and he has showed me every one of them when I tell him it’s bothering me and it’s absolutely nothing but regular friend conversations. I have a best guy friend that I talk to every single day I have since high school so I know the opposite sex can be friends but it’s my fear of the “what if’s” what if she does try to make a move on my husband. What if she not happy in her relationship and wants my husband because she sees what we have. All of these “what if’s” in my own head are causing an issue in my marriage. I trust my husband fully and he has given me every single reassurance and showed me everything but in my mind I’m so terrified that she will try something again and I hate that I can’t let it go! What can I do to just let the past alone because neither one of them have given me a reason to the other wise since that one time she did that years ago but absolutely nothing since then. I really do like her as a person and we can sit and talk even text and talk but I’m so sick to my stomach anytime I know they talk and my husband has done everything he can to show me that it’s literally just a friendship but I’m so caught up on the what ifs that I can not move past it. I need any advise that might help me move forward!