You are always allowed to ask questions. Your husband should be able to communicate to you what he is comfortable sharing and what he isn't, and hopefully the two of you can agree on a reasonable level of information sharing.
Same for details. If it makes you feel better to know, then make him aware of that. Sometimes people think their partner is more comfortable not knowing, and thus will keep the details to a minimum. Make sure to let your husband know what you need/want/expect from him at this level.
Insecurity is built into our monogamous culture and our fear of loss. Only you will be able to figure out what all your individual insecurities are and find ways to combat them. On some levels, you may have to do self-work (like catching yourself in negative thought patterns and replacing them with more positive ones) and on some levels you can ask your husband for support (you two have a dedicated date night a week no matter what, or he always calls to wish you a good night no matter where he is, or whatever reasonable expectation you can get from him for reassurance).
This can be difficult for many people. Ten years older doesn't have to mean you are less attractive as an overall package though. A younger person may be more immature and not have the same kind of life experience you do. Your husband is with you for a reason. He loves you and he is attracted to you.
New relationships are exciting. New relationship energy (NRE) can be very overwhelming and scary for people. It was scary for me when I experienced it with my boyfriend and it was scary for my fiance as well. We didn't know exactly how to handle it or what it meant, but we kept communicating with each other and made sure needs were being met and that my fiance and I made time for each other no matter how excited I was to see and spend time with my boyfriend. It is intense, but it can never last longer than about a year or two, so it is good not to make the mistake of thinking it is a better or more passionate love that is budding just based on the intensity of NRE.
If your husband identifies as poly, this should not happen. If the two of you are to drift, it shouldn't be because something better came along like he is trading you in for a different model. If you two drift due to other reasons (fundamental incompatibilities, one of you completely loses interest in the other) it doesn't necessarily mean it was because of the new person.
Communicate to him and make sure you are being honest with him and yourself about your needs, wants and limits. Don't hold things in and wait for him to read your mind. Make sure to set boundaries that the two of you can agree to and that you can be comfortable with. Be willing to do some of your own self work AND call him into account if you think he is not being ethical or fair.
Don't be mad at yourself. These are natural human emotions and it isn't your fault you are experiencing them. The important part is how you act on them and the behaviors you choose to partake in.
As far as what to do with yourself, you can take some time for something you enjoy on your own (reading, watching a TV show, etc.) or pick up a new social hobby that you have been wanting to try (dancing, horseback riding, pottery). Take some time to reconnect with friends you haven't seen in a while. The possibilities are endless. Right now it is rough because you probably have some anxiety surrounding it, but if you find something that can be fun and relaxing for you, then you might be surprised at how little you worry about it over time and come to enjoy the time you have for you.