At my gym we just have an entire rack of dumbells and up to about 50lbs or so you can choose between thin-grip and wide-grip but after that 50lbs or whatever it stops and you only get to choose...
I'm only 6 weeks in but in 1 month I went from failing at 115x6 to failing at 155x6. My goal is 225lbs by christmas but I know that might be pushing it.
You also might just have bad shoulders. I have a friend who's been rehabbing his shoulder for months but every time he tries to bench press he feels it gimping out.
Same happens to me when I give 110% and still push harder. My entire shirt is usually drenched in sweat too. Probably has something to do with me being a fatty right now.