It's normal as we get older for the center of balance in your body to change, especially in women after they have had children. I went through this, riding bareback everywhere and being willing to ride anything I was put on to having an actual panic attack when I went to mount up. It sucked... I love to ride but I was terrified to get in the saddle.
What helped me was mental visioning, just taking a quiet moment to sit and "ride" through the ride in my mind imagining it going smoothly and perfectly with good balance and good behavior from my horse. What also helped was some conditioning with myself to rebuild my core strength. These two simple things got me riding again, but it took a good trainer to yell at me enough (ha!) and keep me reminded that I was fine, to relax and to breathe. Minus a trainer a good friend that is there to spot you and just help keep your mind right will help.
A poorly fitting saddle for you or your horse can also make you feel insecure. Keep in mind that a saddle that you loved a few years ago can feel vastly different as you get older so it has to be comfortable for you AND your horse.
Great advice in prior posts as well, just some things I went through myself and still struggle through especially on a froggy brat that wants to test me. A lot of it is mental, just don't let it take over your thought process and don't forget to get out on the trails some too with your horse. It's the best way ever to build or rebuild a rapport and help with your own confidence and balance as well.