OP, I could have written your message, twice! I had both children suffering from persistent colic and it was hell, absolute hell and yes, I too hated them (even though I felt so overly protective of them).
With my eldest, it took until she was 14 months old to realise that she wasn't digesting fiber properly. The poor thing had horrible stomach cramps and the only thing that helped her was drinking water which she did in massive dose (5 bottles per night). It was worse at night and she cried every single night at some point. Like you, my GP wasn't interested and put it down to be being an anxious mum. It took me doing diary to put two and two together. I changed her diet and gave her LESS fruits (even though because she suffered badly from constipation was told the exact opposite) and suddenly she became a happy baby.
The experience was so exhausting and frankly traumatic that I wouldn't contemplate trying for a second but everyone told me that second babies were easier so I went for it... and the first few months were even worse than with my DD. Their dad worked away so it was me on my own, no family to help at all and some days, I felt so helpless I had the phone in my hand, ready to contact social services. Like you, I had tried absolutely everything to obsession, but nothing really helped. I was told it would pass when he turned 6 weeks, it didn't. Then I was told colic is definitely over at 12 weeks, it wasn't. Then it was 4 months, it wasn't. Finally, at 5 months old, at the absolute end of my tether and no-one understanding how desperate I felt, I took him to an osteopath, even though I didn't believe in it at all. The guy did is thing and did warn me that it could get worse before it got better. Sure enough, the next two days were worse than ever and I was ready to sue the guy (as I had to use my savings at the time to pay for it), but by day 3, the crying stopped and that was the end of it. I will never know if it was due to the intervention or just that it naturally suddenly stopped, but my god was I relieved.
They are now 18 and 15, the elder given an offer to go to medical school, the second doing really well at school. I couldn't imagine my life without my two amazing children and even though at the time, each day left like a year, I would do it all all over again to enjoy the amazing years I've had with them.
Saying that, some memories, for instance some songs that their toys played will still make my heart beat faster reminding me how distressed I then felt.