Since this sounds like an experienced horse who has been ridden out on trails, I'd say you are facing a situation where you need to do a little reprogramming that even in this new environment, riding out with you (even if it's alone) is an interesting experience that your horse wants to do. I agree with the advice above- it will be a slow, patient process to get back to riding her out alone. You are worried about her reaction, that's part of the equation. As you have more positive experiences together again, your worry will decrease as she becomes more cooperative.
Since you say she is good being led, I'd start by tacking her up but just leading her from the ground. Walk out the gate and a little farther, then come back and call it a day. Gradually increase the distance you go leading. If it's going well, you might end up walking her from the ground for a km or two, the goal is that she is calm and willing, and like @Avna
said, keeps moving forward when you ask. You can even strategically place little "rewards" along the road/trail where you're walking so she becomes more eager to "explore." I don't know how long you'll have to do that, it will depend on how you perceive her willingness- maybe a couple of days, maybe a week, who knows. If music keeps you calm, bring something with you in your pocket to play music that makes you feel happy and confident, and really focus on your breathing and letting go of tension if things aren't going well. The stakes are really low- you are taking your horse for a short walk, and when she walks nicely, that's it. The goals are small at first.
After I could go for a nice handwalk, I'd get on and go. She might ride right out without resistance. She might resist, and then I'd just start the process over again- mount up, go past the gate calmly and turn around the first day, work up to going farther from the barn with her being calm and willing. Don't get greedy at the beginning; if she's improving, let that be enough and keep building from there.