you've only tried a slim fit in size 15. if you had tried a 14.5 slim fit, things would be simpler. as it is, we can't be certain that sizing up in extra slim fit will solve your problems. the collar will definitely be too large, the chest may not be big enough, and the armhole may still be too small. collar size is the last thing you want to mess with, so you should be finding out what the armhole and chest sizes are in size 14.5 by trying other fits.
And why would I want to try a looser fit then slim fit if I think it's to big?
I don't think a 14.5 has slim arms when 15 slim fit has really really wide arms. If I buy a 14.5 slim fit I would have to take it to a tailor and adjust the arms width, which I don't wanna do.