Hi
I know just what you mean. My ds is now 12 months but at 4 months I used to spend the days in tears as he would not sleep more than 30 minutes at a time in the day. Like you I would seem to spend a lot of the day trying to get him to sleep only to have him wake up 20 minutes later (especially if he fell asleep out in the car or pram). Walking in the pram did not work - still doesn't unless he is really really exhausted and then only 40 minutes max.
He was always exhausted by 5pm and would often be asleep for the evening between 5-5.30pm. He hated being rocked or held too tightly. I spent about 3 months (betwen 3 months and 6 months) holding an absolutely screaming baby every evening for between 10 and 40 minutes before he went to sleep (I just couldn't leave him in the cot as his screaming was so bad so holding him was my compromise though the scremaing still happened). He didn't have colic he was just tired, walking and rocking just made it worse as he'd go silent for a while then start again - and this went on for hours.
He was so tired and overstimulated he just couldn't shut off. He even had these tendences as a newborn (no evening colic though, he slept really well between 6pm and 10/11pm - after that is another story - he was up every 1-2 hours).
At abut 5.5 months I'd had enough. I realised that I was carting him everywhere and expecting him to sleep where we were. Well many babies can do this but mine just can't. So I resigned myself to imposing a routine (against my nature) I had figured he would come up with his own routine but he never did.
I woke him at 7am if he wasn't awake already. by 8.30am he was ready for a nap again (sometimes he would be ready at 8am - remember he had a large sleep debt and was exhauseted,). I realised that he needed a nap every 1.5-2 hours. That would mean sometimes starting soothing him only an hour after he woke upp so that he would be asleep in time - especially in the morning when he weirdly seemed to be most tired.
Over time this period extended but even now at 12 months 2.5 - 3 hours is the maximum he can stay up without getting tired and in the mornings he sometimes wants to sleep after 2 hours even now.
As you can imagine this was very restrictive as after feeding and changing there wasn't much time to do anything before he needed to be home for a nap again. I almost went mad from boredom for a while BUT within a couple of weeks I could see he was so much happier that I just did it.
My life went on hold for a while (I have to admit I found it REALLY difficult but it was for his health and my sanity.
Even now I don't often contemplate a whole day out as this means he misses his naps and gets really cranky - though he is managing this better now so we do break his routine maybe 1-2 twice a week maximum.
To get him to sleep was easier if I caught him before he was really tired but I still had quite a lot if crying. I would check on him periodically. I only picked him up if he was really distressed. Even if I did pick him up he would usually keep screaming - so I didn't do this often. In the end it was time and the willingness to be consistent that worked.
Now he is a much better rested baby and happier all round (he has been since about 7 months old).
I'm sorry I don't have an easy solution but you may find as he gets older he improves. Most babies cry less and sleep better at 6 months than as newborn (just not mine !!)
Anna