I'm jealous and it makes me depressed

My SO has been married before and has two kids with his ex. They were together for 10, almost 11 years, which is a pretty decent amount of time. When he talks about marrying me and says I love you more than anything and you've won my heart more than anyone and you're everything I've always wanted in a woman I get upset because clearly he loved someone enough to marry her before and have kids with her. I feel a little mislead. Clearly she was everything otherwise you wouldn't have married her or had kids with her. I lose sleep over it. It makes me sick to my stomach. I feel like I came in second place. Someone has already had his heart so I don't feel as special. And seeing as he once loved her enough to marry her then divorced her, who is to say he won't do the same thing to me? Am I wrong to feel this way?