We lucked out with our daughter - she's a fabulous sleeper so far (but we are only 5mo so we'll see). But she sucked at breastfeeding. We were so terrible at it she ended up in the hospital because she was basically starving. It seems like every baby struggles with something - sleeping, breastfeeding, spitting up/acid reflux, etc. Sounds like you got the short end of the stick with sleeping, which is extra tough because sleep deprivation is a form of torture.
Our method we used to try to get her to sleep longer (when she was about 2.5-3 mo) was to leave her in her cosleeper beside the bed when she fussed, and just reach in, pop her pacifier in, and then let our hand rest on her chest. That way, she was soothed, she knew we were with her, but we did not lift her, get up, or turn on the lights. Basically we just tried to soothe her in a way that didn't overstimulate her at all or send the message that it was anything but time to sleep. We rarely need to do this now. Obviously this method won't work for you since she is no longer next to the bed.
I'm sure you already tried all of this, but what I've heard is that the key is not to overstimulate a baby when it is sleep time, even if they are fighting/crying. I've also heard that some babies respond really well to strict bedtime rituals - the same pattern before every nap and bedtime. Think change, bottle, lights low, and the same song every time. No changes, no switching stuff around, and no giving in when they cry (because if you go in and out, then you are changing the ritual). I don't know if it will work, but it might be worth a shot if you are willing to let the kiddo lose it until they fall asleep for the first few times. Hopefully after a few days the baby will be looking sleepy by the end of the ritual and will find it easier to doze off in the crib.