She could be over-tired. Sometimes, if the baby isn't getting enough sleep during the day, it can make it harder for them to get to sleep at night.
Do you have a "going to bed" routine? Try starting a little earlier in the evening with a bath and maybe some story time. If you are nursing, a lot of time it really helps to nurse your baby to sleep. Also, co-sleeping can really help you and your husband get more sleep because once you get your baby to sleep, you can catch her before she wakes up fully and gets upset and you can usually get them back to sleep without having to get out of bed.
Also, try starting her sleep routine before she gets fussy. Watch her and she'll give you cues that she's ready to start getting ready for bed. If you wait till she's fussy, it'll be that much harder.
P.S. I also want to encourage you to follow your instincts. A lot of "sleep training" methods will encourage you to let your baby cry it out, even if you have a really hard time letting them cry. The instinct to respond to your child when she's crying is there for a good reason. When babies who cry it out quit crying, it isn't because they've learned to self-sooth. It's because they've given up thinking that their parents will come to help them.