First of all, I don't think your horse is thin, however she could use more muscle in her hindquarters!
I was having a similar problem with "my" (he is my trainers horse and was used a lot for beginner lessons, but I've since taken over and have been his primary rider) 13 y/o TB. Building up a top line is hard and takes a LOT of work! Over the past 3 months I have been working with Blazer 5 days a week and can definitely see a difference. I wish I would have taken before and after pictures, but of course I didn't stop to think of that at the beginning of our journey.
IMHO, the building of a topline starts from the horses butt. The horses hindquarters are their engine! When a horse powers from behind it results in lifting their back and then their head comes down BECAUSE their hindquarters are engaged and their back is lifted. You dont want to pull your horses head down, but rather have them push from behind which results in a lower head set. This is much, much harder than it sounds (at least in my opinion anyways!). What has worked really well for Blazer is that I lunge him in side reins for about 10-15 minutes EVERY TIME before I ride him. I will lunge for about 2 minutes each direction without side reins so he can get a little warmed up and then clip them on. I DO NOT put the side reins on tight, they are rather loose to encourage him to stretch down and out. I encourage him to engage his hind end and lift with his back legs by raising the whip (sometimes I have to crack it), and then gently tugging and releasing on the lunge line so that he uses the extra energy to come UP rather than go faster. I also do a lot of transitions on the lunge line.
Undersaddle I do a LOT of transitions. Transitions within the gate and transitions from gate to gate too. I do a lot of circles and changes of direction. Again, IMO, this helps keep my horse's hindquarters engaged and prevents him from falling on his forehand. I have also been doing raised cavelitti to help him really pick up his back legs and thing about what he is doing. I have noticed an a HUGE difference since incorporating caveletti into his workout.
Also if there is anyway you can ride her more frequently, or maybe come out an extra day and lunge her with side reins that will probably help her build muscle too.
Good luck! It's not easy, but it is definitely do-able and SO worth it!