Firstly - you're not a crap mum. Sometimes babies cry and all you can do is hold them - you can't stop them crying, you can just let them know that they're not alone.
And this may be small comfort when you're going through it, but there is a sleep regression around the 4-5 month stage when habits change and they can start wanting more milk and sleeping less and being grumpy. This is also when teeth can start to move beneat the gums, getting ready to come out - you might not be able to see anything, but it doesn't mean teethign hasn't started.
DS1 was a reluctant napper (hated naps, used to have to force him to take them) and initially would only sleep in buggy if it was outside in motion - so it was brisk (you had to walk fast for him to nod off) 60-90 minute pram walks for me, we'd get home, he'd wake up, I'd be shattered....
Inside, we'd have to put on a CD (background noise) and push him round the room for up to 20 minutes before he'd nod off, then keep watching him and pushing him as he'd stir every 20 mins. He'd be crying for the first 10 minutes or so of us using the pram, before he would then go quiet and get ready to sleep. It wasn't fun for any of us 
I did find, that when he was crying hysterically a change of scene could help stop him (by distracting him) so if he was in one room, taking him to another differently lit room, or opening the door and taking him outside, could put a stop to his crying and then gave me an opportunity to do somehting with him. In honesty, he was rubbish like that until he started crawling and then as he got physically more tired, we still needed the buggy for him to go to sleep during the day, but he would nod off quickly without a fight. Some babies are just harder than others. Flip side is he's now 4, sleeps like a log and has a good 11/12 hours a night :-)
As for the 'everytime I think I've cracked it, turns out I'm wrong' that is babies all over, very few babies are the perfect babies their mums say they are, babies rewrite the manual all the time and you're left feeling hopeless - those are normal everyday feelings you have, every mum feels like that - which isn't to diminish your feelings, but just to let you know you are not alone.
Definitely try and find out via your HV what groups etc are available locally as meeting up with other mums and hearing them moan about their babies can be remarkably therapeutic :-)