"Can you please give advice as to what to do? My little boy is 18 weeks old and will only fall asleep if held/nursed or pushed in his pram. If I try to put him down awake in his cot and sit with him to go to sleep he starts to scream and will eventually get into a state of some distress. I can not bear to see him cry in such a way. Can you please give me advice as to what steps I can take to learn him to self soothe without causing him any distress. Any help would be very much appreciated."
I really hate this idea that a baby should be able to go down in a crib awake and fall asleep on his/her own at only a few months old. I think it's highly unrealistic and causes a ton of stress for parents who think there's something wrong with them because their baby just won't do it. Of course there are going to be some babies who will be able to go down awake and put themselves to sleep from Day 1 (or a couple weeks old), but most kids really have to be taught how to go to sleep.
It seems to me that it helps to figure out exactly what your sleep goal is for your child. If your goal is to get your child to go to sleep without intervention from you at the youngest age possible, then it might be worth it to you to aggressively pursue training your child to go to sleep from an awake state, no matter how long it takes. If your goal is to condition your child to associate falling asleep with positive, stress-free time so that they'll fall asleep easily under various conditions for the rest of their life, then pushing your child to learn to self-soothe before the child is ready to is going to be counter-productive ultimately.
The advice some experts give that kids "have" to be be able to go down awake and get themselves to sleep by a certain age or they'll never do it is simply wrong and IMO is a scare tactic. Experts who can create fear and feelings of inadequacy in the people who read their books will get followers who buy more books and try even harder to live up to the experts' dogma. But think about it: Do you know anyone who needs her mother to come over and rock her to sleep as an adult? (And that creepy Love You Forever book doesn't count.)
OTOH, I do know several parents who have tried so hard to get their kids to go down awake (because some expert said they had to) that they ended up with 2-hour ordeals for naps and bedtime. The parents were frustrated and exhausted and felt inadequate, and the kids were jittery and exhausted. They all would have been far better off if the parents had just listened to their kids (instead of a book or TV show or website), rocked or strolled or nursed or pacifiered or whatever to sleep, and then adjusted to the next stage when it happened in another few weeks. You can't put a price on mental health, and you can't put a price on everyone getting some sleep.
As with everything else humans have to learn, self-soothing to sleep can't be taught until the person is physically and emotionally ready to learn. That point is different for all kids, but gets easier as a kid gets older. There are many babies who need to be rocked to sleep at 4 months who can soothe themselves to sleep at 6 months. Or who still need to be rocked to sleep at 11 months but can go to sleep on their own at 14 months. Some kids need some help going to sleep for years, and then one day just say, "Goodnight, Mom!" and go to sleep. It doesn't mean there's anything wrong with them--it's just the way they're wired. If you give them what they need, and every so often test to see if they're ready for the next step toward independence, separation and self-soothing will fall into place for them when they're ready for it.
It sounds like Amanda's son is just simply not ready to go down awake yet. So I'd stop trying completely at this point and revisit it in a month or so. In the meantime, I'd start laying the groundwork for teaching him to go to sleep by putting some kind of musical lovey in his pram with him while he's strolled to sleep. That way he'll start to associate this lovey and its music with falling asleep peacefully and easily. (This is also useful for babies who nurse to sleep--just snuggle the lovey in with the two of you while you nurse to sleep for a few weeks.) Then once you do go back to putting him down drowsy (don't try to go cold turkey to putting him down awake unless he's just passed one of the developmental leaps, stick to putting him down veeeeery drowsy) the music and lovey will help to bridge that "I'm half awake--should I go back to sleep or wake up and start crying?" gap.
Try again in another month and he'll tell you if he's ready for it. If he protests a little, keep trying, but if he becomes scared and worked up, back off again and just give it a little more time. All kids eventually get there, and if you push gently but not past what the child can take you'll end up with a kid who goes to bed happily and easily even at the age of 5 or 8 or 15 or during menopause.
Learning how to fall asleep is one tiny part of the main developmental task of the first year for babies, which is learning to trust the world and his/her parents. It can be a really stressful stage for parents, because it means you have to respond again and again and again, even when you're beyond exhausted. Please don't make it worse on yourself by believing the "don't let your kid do that!" hype. If your child is helped by a pacifier or music or a lovey or co-sleeping or the crib aquarium thing or nursing to sleep or strolling to sleep or patting or whatever, just do it. It's part of the process of teaching your child that falling asleep is fun and easy. When the child is ready to move on from the prop, you'll be able to take it away, no matter what that lady at the grocery store says. It won't happen easily in one night, but you can definitely do it if you pay attention to what your child can handle and give yourselves enough time to adjust to the new thing.