Dealing with Overfitting
Yep - you've overfitted !

Can you see what your accuracy is as a function of the number of training cycles? It would be interesting to see how that looks.

Is there any way you could increase your data? 15000x300 isn't a huge amount of depth for a neural network, considering the width of the data set.

You could look at the impact of increasing your regularisation parameters, or decreasing the depth of your network