My daughter is eight months old. We DID cosleep, and I had this very problem...all night long. It wasn't until I put her into the playpen to sleep (we don't have a crib, but a play pen in our room that I had intended to use for her) that she started sleeping in 3-4 hour stretches...and I think that's pretty good.
Honestly, I think it's unrealistic to expect an 8 month old (or 6, or 4) to sleep through the night...I've never personally known anyone who had a child who did it.
But I hear you...it got to the point where I just wasn't sleeping well at ALL (even though she was)...
My advice is to cut back on naps, slowly, so she's down to two naps, and NONE of them after five o'clock. Keep her up and play and be active after dinner, hold off ALL feeding after dinner until you'd like her to go to bed, and then give a bath, then feed. (Are you feeding very frequently during the day, in shorter lengths? She may be used to taking "too small" meals more frequently.) I usually have my baby down by this method by 7 or 8.
She wakes up at 10 or 11 to eat, I feed her, and put her back...and then go to bed right away! She's usually up again at 2 or 3, and if my husband is awake, sometimes he will give her a bottle. In fact, I got so stressed out by exactly what you're going through that I bought a can of formula and said "please do this only in the middle of the night."
After about a week of that, for some reason, it seemed like she started stretching it out longer...so we quit giving the formula and I just get up to feed her...turn on the instant netflix, watch something only I like, and nurse her back to sleep. Then I go back to bed, until I have to get up at 7.
Just try it...it's what worked for me. I cut back on naps, no nursing after dinner until after bath and it's bedtime, fed again at my bedtime, and once during the night.
I still only get 3-4 hours of sleep at a time, but I feel like a totally refreshed woman.