Absolutely doing nothing wrong, you are doing what comes naturally and that is the best thing that you can do but I completely understand you frustration. My DD who is also 5mth old has always been hard to settle and these are some things we are trying.

fIrstly I know you said you have read heaps of books but have you read the No cry sleep solution. We are working our way through it at the moment - used it for DS too.

Secondly - we have been spending lots of time on getting daytime sleps sorted as better daytime sleeps = better night time sleeps
- nice calm environment before bed
-we made her bed time ealier - we were going to bed at 7:30-8pm now at 6-6:30pm
-we wrap occasionaly but sometimes it makes it worse as she gets frustrated
-temperature (too hot/too cold)
-massage
-as another poster said is there any colic/wind/teething etc
- we considered getting a hammock but her sleeping has gotten better recently so haven't resrted to that yet
-settling on tummy

not sure if any of this is of help but hopefully both of us can sort it out and get a decent nights sleep.