I know exactly what you are going through but from another perspective. I have a husband and have recently started dating someone for the first time and I am constantly worried he's going to become insecure and think he doesn't want to do it any more. I think feelings are normal and worry is normal. I can't say im worrying any more or less than i did when i was only with one guy. As long as youre happy then your "cut out" for it.
you have to take each relationship as they come and understand youre part of a bigger picture. Openness and honesty plays a big role so be sure to share what youre feeling with the guy