Hi A.,
Sounds like you have already received lots of advice, but since your situation is so similar to ours, thought I'd let you know what we're doing.
Our little girl is now 8 months old, but she stopped napping on her own at 4-5 months and would cry all evening because she was so tired. She slept through the night at 2 months (lucky us!) without any sleep training, and only stopped at 7 months for a couple of weeks when she was teething.
I spent a few weeks of staying at home every day and just focusing on getting her to nap in her crib, following some tips from Elizabeth Pantley's "The No-Cry Sleep Solution". As soon as she showed even subtle signs of tiredness (in her case, eye rubbing, ear pulling, head scratching, slight crabbiness or getting quiet), usually about 1.5 hours after waking in the morning, I'd nurse her until she fell asleep, hold her for a little bit and then put her in her crib. It took a while until this worked, picking her up when she would wake up and cry, and start all over again. It almost drove me crazy because it was so time consuming and frustrating, but it did work in the end, though she still would only sleep for 30 minutes. I know 30 minutes doesn't sound great, but it was a huge improvement.
She would then sleep in the middle of the day for another 30 minutes, either in her crib again or in my lap or in the baby Bjorn if we were out of the house. I also laid down with her in our bed in the afternoon (nursing her to sleep) when she'd sleep 1-3 hours. If she woke up in that time I'd offer her the breast and she'd suck herself right back to sleep. Again, time consuming, but at least I got a lot of reading done and a few naps myself! And it made a huge difference in her evening mood. It seemed to be just what she needed.
I know, it is ridiculously time consuming and takes a lot of patience (I kept telling myself that she'd be off to college before I knew it and I'd wish I could hold her and nurse her again), and I have no idea what parents do when they have more than one child or work out of the home. But we had a nice little nap schedule that was lead by her. It was great because I could plan my day around it and she was in good spirits until we put her to bed.
That said, at about 7 months she stopped following her nap schedule, I think because of teething (she also started waking up every 2 hours at night!). So keep in mind that as soon as you get a schedule worked out, it might all fall apart as your baby grows and changes. But we're just getting back to a similar schedule and it's MUCH easier this time since we'd already been through it before. And the hope is that her naps will get longer and longer and she will be able to do the long afternoon nap by herself.
One more thing to consider is that we found there's a middle road between responding to every peep our daughter makes and the cry-it-out method. When she started waking at night we couldn't stomach the CIO, so were up with her for a week or more, every 2 hours, usually for an hour each time trying to get her back to sleep. That just wasn't sustainable so we did our own "complain-it-out" method where we listen carefully and let her complain a bit and she'd eventually fall to sleep on her own. I still felt guilty about this, but if we jumped up to get her as soon as she made a noise she'd start to really cry when she saw us (and fall right back to sleep in our arms, so we knew she wasn't suffering from pain or sick or anything). We'd go to her if she sounded really upset, but let her whine a bit if she just sounded kind of annoyed, and it worked! The very next night it was better, and now she's sleeping 9 hours, wakes for a breast feeding, and then goes right back to sleep for another 2-3 hours. Though, we've only gotten this to work at night, not during the day. But it might work for you for naps as well!
Hope this helps a little. Most of all, I think it's helpful to realize that our babies grow so quickly and will be sleeping through the night eventually, sooner than it seems. Hang in there! I hope things get better for you and your baby very soon!