Eat more. Eat like 5 meals a day, put on some freaking weight for a start
Lol I'm surprised her legs were long and strong enough to triangle
Work on triangle escapes for a start. The fact that two were mounted triangles leads me to believe you're pulling over to the side too much when trying to escape so you're falling onto your back.
Protect your arm so you don't get armbarred, then posture up, pull up to create a bit of space then twist with your body, so instead of falling on to your back, you come down and take side control. (Or, as soon as her leg comes up over your arm to prepare the triangle, twist and push both her legs to the side of the arm her leg just came over and pass her guard that way)
Get your instructor to show ya, only so much I can do with words
At the end of the day, yeah you don't see many 26 year olds losing to 10 year olds, you did have a 1.5 foot height and 45 pound weight advantage
But if that one month she practiced triangles and for your 6 months you didn't practice triangle escapes as much as she practiced triangles then it follows that she'd be able to catch you with them. I mean you see people with bigger physical disadvantages out rolling larger opponents in BJJ that's a fundamental idea of the sport
Tl;dr the answer, as it almost always is, is to train.
P.S and eat. Get that weight up. 😎