We read, all the time, that relationships should go as slow as the slowest member. Right now that person is you. Whether she likes it or not you ARE a part of her new relationship. You need to know your feelings are valid (they are btw) and that she's hearing you when you voice your feelings or anything else.
Sit her down once the kids are settled for the night, have a babysitter, what ever and discuss this with her. Let her know how you're feeling. Write it down if you need to.