Well, insecurity is a purely internal thing. There isn't anything that your boyfriend can do that can cure you of your fears. I was very insecure when I started dating my husband too. I realized that it's pretty friggin easy to tell if someone is messing around and no one can successfully pull it off unless they are an actual sociopath, and even then, you can tell that he's a sociopath by his lack of moral compass and proclivity for repeating mistakes. You even said that you pretty much figured out that your one boyfriend had something else going on. It wasn't rocket science.

I think you need to start trusting yourself more and reminding yourself of the good that you bring to a relationship and in general. If you trust yourself more, you can learn to start to trust him more. If you do have disagreements and conflicts, trust that you will be able to handle it and work it out with him. If things don't work out for whatever reason, trust that you would be ok being single and you won't spontaneously explode into powder. It's a gradual process, but worth it in the end!