I wrote a similar answer a while back that was from the perspective of a player wanting to help a struggling GM who was heavily railroading her players. The advice in that answer basically boiled down to the following:
Model good behavior to show her that loosening up doesn't mean the game will be ruined
My (educated, because this was me) guess is that she's worried if she lets you guys run amok she's going to get overwhelmed quickly and forced to improvise more than she's comfortable with. I don't know anything about 13th Age in particular, but generally if you're working with a new system you're unfamiliar with it can be a lot to hold onto mentally. Maybe she's just barely gotten enough material prepped for your session and is worried if you go off rails she'll not be able to keep up.
While some of this fear is difficult to get rid of because it really does just require a lot of familiarity with the rules and setting, some of it can be mitigated with good player behavior.
I had similar concerns when running my first few sessions of D&D because I was unfamiliar with the system and unfamiliar with my players' characters. I didn't know what they would want to do or where they'd want to go, so I heavily railroaded to ensure I was never caught with my proverbial pants down. The issue was, I took a lot of the fun and spontaneity out of those sessions because they never got the satisfaction of making a choice that mattered.
My players brought up my railroading to me and I actually asked about it on here and took some time to familiarize myself better with a larger range of possibilities, and slowly managed to railroad less with practice, all because we'd had a conversation about how much it bothered them.
The rest of this answer assumes that you're having conversations with the other players and that a) they also see the problem and b) are willing to help out where they can to fix it. If the rest of your group's not seeing an issue, you may not have many options.
Have a conversation before you start
Ideally this would happen outside of your typical game time, and face-to-face. Let her know that you've seen her behavior change between games, and that the freedom offered in the previous game was enjoyable. Be sure to not make it an accusation - rather than focus on how it's bad now, mention what was good before. It also helps that you've been playing with her a while in other games with other GMs as you can reference those as examples where you both went off the rails and had fun.
Tell her that you (and I imagine also the rest of your group) has more fun when the railroading is minimal. Let her know that you want to help her get more comfortable, and ask if there's anything you can do to help. Perhaps she's well aware of the issue already and already has ideas for how it can be fixed. If you're lucky, this might be all that's needed to fix it.
However, if she is resistant and doesn't acknowledge that there's a problem, you're left with two paths: continuing to play and trying to teach her over time that easing up is okay, or finding a new GM.
1: Keep playing
If she seems receptive to change, it's time to show her through your actions in-game that things won't get ruined if she doesn't railroad you. This is where the points in my other answer come in, but the highlights are:
- Draw attention where it's needed. If she mentions an obvious feature or clue, make sure to pay it the attention it deserves. This is not the time to walk straight past the elaborate, well-detailed tower to go rifle through boxes. Make sure to acknowledge that you see the tower, are planning to enter it shortly, but would just like to check out those boxes first. A GM doesn't know what's going through your head and can't always predict your actions, so you need to make an effort to make your intentions clear.
- Model good behavior. It sounds like you all have been playing together a while so this might be less of a problem, but until she's more comfortable and has relaxed a little, make sure not to go too far off the rails (literally or emotionally). I know it's not as much fun to not be a bloodthirsty gang of wild monkeys, but having predictable responses to situations will put her at ease.
- Give her feedback. Try to narrate more of your thoughts out loud. Have more planning sessions in front of her. Make sure she's aware of what you're thinking whenever possible. This is pretty similar a point to #1, but less about acknowledging the current situation and more about the future. If you think you're going to want to go somewhere next session, let her know ahead of time so she can plan.
- Intent over action. If all these points sound really similar, that's intentional! Again this is about telling her what you want to do, but on a much smaller scale. Instead of telling her you want to climb a wall, make sure to mention why you want to get up there. The less of a mystery her actions are, the easier it is for her to ease up and let you try things. In my group, I always encouraged my players to tell me why they wanted to do something. It changed the conversation from them blindly trying to scale a tree, to scaling the tree because they wanted to jump down on a guard walking by. That little difference made it much easier for me to either approve the tree-climbing or shut them down before they started. Either way I had insight into what their intentions were.
Ideally the more predictable your actions are, the more comfortable she'll get and the more she'll be able to ease up on the plot.
2: Move on
If you've had a conversation and maybe even given #1 above a shot for a few weeks and none of it is working, it might be time to cut your losses. Maybe you can sit out of sessions where she GMs, or find a way to live with her style that won't drive you up a wall, but in that case she might just never be comfortable enough to give you the play experience you're looking for.
