S.D.
For the first year, a breast-fed baby is considered to "sleep through the night" at 5 hours! I know it's hard, but I promise you don't have to force it. She'll sleep longer when she's ready. My first slept through the night around 7 months after we started solids. My second didn't until around 9 months.
Is she still in your room? She might be bothered by sounds and you breath, turn over, etc. You could try putting her in her own room for a weekend and see if that makes a difference.