Maybe you are just getting too excited? Happens often to beginners. Just be calm and keep practicing, i'm sure they will become longer with practice. And if you can't open eyes in the dream, try using a bit of dream control and take whatever blocks your vision off your head, like a blindfold or such and expect vision to return. Also it helps to not overthink about it, once i had problem with dream length and was trying to figure out stuff, but then i stopped worrying about it and the length of my lucid dreams stabilized.
And yeah, stabilization techniques can help, check out that tutorial JoannaB linked to.