Originally posted by Pit Chick
Get the terrier spayed ASAP. This should greatly minimize these attacks. The terrier is a mature adult now and is acting like one. Same sexes in dogs won't always get along, especially when one is fixed and the other isn't. The terrier is hormonal and territorial. By giving the terrier more of your attention you're spoiling her and she doesn't want to share you with the lab mix. These attacks seem to be happening when you are around, so you are what the fights are all about.
Actually when it comes to females, the majority of the time, spayed females do not get alone with each other more so than any other combination of sexes, neutered or spayed or not.

If it is bad now, it may get worse after both are spayed.

As stated before terriers can be... well just that terrors LOL. They are great dogs don't ge me wrong, but they can be a difficult breed.

I would contact a behavorist asap.

Good luck