Number 1 thing is hang in there, we had worlds worst sleeper first followed by worlds best sleeper second, for no apparent reason.
Best advice I can give is don't worry about what any book, expert, parent, grand parent or self appointed guru says. I remember the sleep school nurse saying to my wife, "I'm not sure we can help you, but you've got a very strong willed little person there." I was wondering what to do with my hysterically crying wife at this point.
However at some point we threw out all of the techniques, at about 17 months of age, we gave up, as my wife was now 7 months pregnant, and took him in with us. SHOCK HORROR we slept with our child. ( as mankind has done for thousands of years) suddenly we all slept well. " but you'll never break that" said all of the experts, at age 4 by chance we put our boys in the same room, and 6 years latter all is well. We have had them on their own now for 12 months.
So my point, mate, is hang in there, there is no rule that works for every child. Do what you need, it will be ok. Ignore the parents bragging about their kids sleeping 23 hours a day, and how this somehow makes them super parents.
Hang in there, and obviously have all of the medical checks for reflux and the like.