Don't feel like a failure as you are doing a wonderful job with our little girl. I think it could be the 6.5 month age group where by they don't seem to sleep well whereas previously they did for weeks and weeks on end. OUr little girl also was a good sleeper from 10 weeks up until 6 months or so with no waking in the night. We have just had about 7 weeks of her waking up in the night, crying, screaming - not too sure why. We took turns in rocking, cuddling trying to get her to sleep again, even having to take her out and play with her at 2:30am for one night whereby playtime lasted around an hour! One bad night between my husband and I we only got around 3 hours sleep total (and she didn't sleep well that next day either!). We didn't put her in our bed to sleep with us, as, like you fear, I didn't want to start up a habit which may have been difficult to break in the future. I don't know what the trick of it was, but we finally managed to get her to start sleeping again with only her waking once for a feed (previously none overnight). Now she is on the move in the cot and starts to stir when she has wriggled out of the blankets at the head end of the cot. I don't know what the trick is it could just be a "phase" perhaps that will pass hopefully sooner than later. We now know why she had such a disruptive sleep pattern, she was teething for those 7 weeks and finally now I can see the little tooth about to come through (her 7th one now). It all makes sense now, but certainly not at the time.
You are not alone in what you are feeling and hopefully your little one will settle down very soon for some urgently needed sleep for you both.