I know exactly what you are going through. My baby and I are going through the same problem... except she is 11 months old now! I've been cuddling with her and sleeping with her on the couch for a while now. I used to think it was our quality time while the older kids were at school and my hubby was at work, but now it seems that she needs it or else she can't sleep.
I heard the age 6 months holds alot of window of opportunities as far as switching them to something new, like sleeping on their own, drinking from a sippy cup, or getting off the pacifier. What I would do...should have done... is maybe put her down when she gets sleepy. Let her cry for about 5 minutes then go to her, comfort her, then put her back down. For the first day, just go to her after 5 minutes. Then the next day try going to her maybe after 7 minutes... then the next day after that 10 minutes. I read this in an article.
I purchased a pack and play for my daughter. I haven't timed myself, but I noticed the first day was worse than the days that followed. It gets better. I am also like you in that I really can't stand to hear my daughter cry. It breaks my heart. I feel like its my job to go to her and do what it takes so that she doesn't cry. For that, I read in another article.....try and talk to family or friends on the phone while you are trying to wait the 5 minutes. In the article the woman talked to her Father in law who was also a pediatrician, and she said it really helped when she was trying to let her baby cry it out. When I first heard this article, I was thinking to myself "that's nuts, how do you carry on a conversation with a crying baby in the background?" I can see how this would make it easier for the mom though. Just let the other person you talk to know that you need them for the support. :)
I wish all the best to you.
R. T.