Hi--
I started pat/shh at around the same age with my lo. She is 11.5 weeks now and still has a hard time to go to sleep at times. However, on a good day, I can put her in the bassinet wide-eyed and leave her to fall asleep on her own within mintues. She will usually fuss for a bit but I let her settle herself to sleep. I will only go in if she is really crying. I saw results after a couple days, but it took about two weeks to really them. But every baby is different, might be sooner for you.
By the way, I don't actually pat. I found that a slight jiggle with my hand on her chest worked better for us. I sort of rock her side to side and that will usually calm her down.
Like you, the second day or so was really good then it sort of regressed and got very difficult. We had a growth spurt and some reflux so that just made everything worse. Someone recommended to me if your lo is really upset then pick him up but I would mostly try to shh her in the bassinet. I even used earplugs, and still do, to soften the volume of her cry. I don't ever leave her but I try my best to either shh louder than her cry or just talk to her and try to calm her that way. Remember, Tracy suggests staying for the whole 20 minutes, or longer in the beginning to get them through the stages. I would stay for over two hours at a time as well. It gets really hard at times and I shh-ed myself to tears once.
I definitely know what it feels like but stick with it. Also, someone told me to try giving your lo play time in the crib so they start to feel comfortable in that space and may start to put themselves back to sleep. I still try to do that now. It seems to work since dd will put herself back to sleep more and more now.
Good Luck.
Lori