Mum2 -
My 3yo is a really bad sleeper - always has been and honestly there is not much we haven't tried. It is not your fault - just how your DS is, don't let anyone make you feel bad about it.
We had similar issues at 18months and then again when DD2 was born.
What you need is to find the best fit for you both - I found trying to leave her with door shut etc only escalated the anxiety on her part. In the end I found that I had to keep her nap in the day shorter, I limited to 1 hr on the days I could bear it (initially very hard to do) and made sure she was up before 3.30pm. We'd head out to the garden/park and do very physical things for an hour - then we'd come home and start winding down. So dinner was 5.30 if possible, then a bath and books and bed - I was strict about timing and she was in bed at 7pm each night. I'd read 2 books in bed, we'd talk about the same things each night so she could predict the routine....I still ask what her fav part of the day was, tell her mine, praise her for something I was really proud/pleased with and then tell her how much I love her. DD was pretty clingy and seemed to be quite stressed about where I was so I explained that I had some jobs to do and then I would be going to bed. Initially I sat next to her reading a book, I explained that I would sit there until she slept but I would not talk to her. So no eye contact etc and no talking....I gradually moved out of the room and into the hall way. It did take some time and she is a little older but it really seemed to elleviate the anxst without her getting superstressed. I would also tell her that I wasn't going anywhere and that even though she couldn't see me I was still there and loved her etc.... this may seem OTT but it really helped as I guess she needed to be able to talk herself through it....
I felt bullied into other options and all they did was make things worse - so trust your gut and instinct and follow it. It is really hard to be patient when all you need is some sleep, especially when you dread what is coming up for the night. Hang in there you will get through this. I used to feel like a failure about DD1 sleep but I look back and wish I had trusted myself as I believe I would have been far more relaxed and excepting of our own situation rather than beat myself up about it. FWIW - DH was and still is pretty impatient about it being 'fixed', not very helpful when you need support.
HTH - sorry it was soooooooooooo long! Perhaps read it to DS before bed, lol!!!