Sweetheart, in my experience, the best thing, and the only thing that will convince you is what you already know is best for you. If that means co-sleeping, then great. If it means finding another arrangement, then great, too. Co-sleeping is really only the best when it serves everyone involved. That said, I am currently co-sleeping with an 18 month old who has been doing the same thing as you describe since the same age, and she often doesn't go to sleep at all until 11pm or later. Last month, I sent in a resignation letter for several teaching engagements that would have been very lucrative and promised to bring in more demand for my services. The hardest part of that for me was feeling like I would be disappointing others by resigning; doing so for the sake of my dd was very easy for me; I knew I wanted to have the ease of simply accepting her way of managing her sleep without the stress of resisting it for the sake of my job (even though I really enjoyed that, too; it was always secondary for me). It turned out that everyone really supported my decision once they knew why and how I really feel about my role in my family and my relationships with the people in it.
So my partner and I devised a way to accommodate dd and us that is unconventional, but very freeing to us and her. We bought a futon and sleep on it in the livingroom. In the evenings, we go about our business as usual, and dd joins us in what we're doing or plays or whatever, and when she's ready to sleep, she usually invites me to come into the bed with her to nurse her to sleep. Sometimes she just wants a cuddle and no nursing. Sometimes she wakes up and climbs out of the bed and wakes right up as if it were daytime, and because I am not resisting this experience, it is such a wonderful time; she's so cuddly and her chatter is extra soft and gentle. She gives more kisses and hugs than during the day (which is already a LOT :), and usually just wants to connect by sitting on my lap for a bit, plays with a toy or book, an she's satisfied an can feel her tiredness. Sometimes she just wants to play, and she does. She's used to it being a quiet time because she has four brothers who are asleep (usually) by 10pm, so she tends to also be very quiet, but of course, not always. Having the bed in the main room makes it easy for us to just flow with things as they are, and me being home helps, too. My partner took a night shift so that he could be here for the day and bedtime stuff, so I must include this enormously beneficial decision that he gives to our family as well. He leaves at 11:15pm.
Anyway, how ever it works best for you, is best for you. With my four boys, the period of back and forth sleep happened between around 9 months and just when I thought I couldn't cope anymore, it ended, usually between 14 months and 18 months. Dd has been gradually shifting into a regular bedtime pattern over the past two weeks, which seems the same as the ones her brothers had when they began to sleep as a routine. Whether or not this is it, I don't yet know, but I don't cope anymore because I have learned some very helpful lessons about myself in the past year, and those lessons have opened me up to the world outside and inside in a way that coping prevented. Now, it is all joy. It sure was very hard for a long time, though. I get that. I really do.
My second son couldn't co-sleep past four months old. It was hard on me to let him have his own space, but he really demanded it, and in the end, I realised that it was me stopping him from having what I could so easily provide if I really saw him. He cannot sleep with any sensory input beyond the stillness of the fabrics and air around him. My breathing, my skin, my movements, the covers when I moved- their sounds, the texture and changing airflow- all completely disrupted his sleep, and he all but kicked me out of my bed to get that across to me and my dp! When we moved him to a sidecar, he protested in the same way, and when we moved him into his own room (in those days, this was devastating to me), and he not only didn't even whimper, he looooved it. He was totally in his glory, and so happy to go to sleep at night in there. I still held him for naps during the day and cuddled with him all the time, of course, but his nights were his, and now he's 7 yrs old, and the same. He loves sleeping near his brothers though, so they all have loft beds in a big "L" in their bedroom. Anyway, it will work out, and you can really save yourself a lot of heartache by just becoming still, observing what is really happening for you, acknoledge and then examine fears if you have any, and the answer will simply emerge, because it's already there.