It’s a wise move to be proactive with strategies for your return to the saddle – kudos! Virtually every rider has had to go through a time of getting back into the sport, so please rest assured that you are not alone in sorting through this process.
I agree with Bernie about making sure you are strong physically before you return, as this will have a positive impact on your confidence. Strong means muscle strong and cardio fit, both will be a boost to your positive attitude and ability to focus in the tack. I also support your idea of groundwork and spending time with your horse to strengthen your relationship and build (or re-build) trust.
I encourage you to really listen to your instincts about what feels right in your first few rides. Be kind with yourself (this is key!) and keep your expectations relaxed. The important thing is to feel positive and happy during your rides. This may mean riding more simple exercises, or just walking and trotting your horse, for example — anything that keeps you feeling comfortable and in control. Having good help on the ground from your trainer will be important, as well as feeling that the lines of communication are open for you to talk about how you are feeling.
I also recommend setting small performance goals for each ride so that you can give yourself positive feedback and notice places where you made progress each day. You want to have places to channel your energy toward that will help you ride well, rather than allowing your insecurities to become distracting. Focus on what you want to achieve (even if these goals are small, like keeping your weight even in your stirrups) rather than getting on for each ride thinking, “I wonder how it’s going to go today?” This is the type of thought that only builds fear and feelings of being out of control.
Most of all, appreciate the opportunity to be back on your horse. Riding is a wonderful gift. Enjoy every moment!