How upright are you when your sitting on your bike and not climbing? If your body position is to upright it’s going to make climbing the steeps hard. It puts more weight on the rear of the bike and your going to want to pull up on the bars way to much when your climbing. You might try a shorter rised stem or a stem with a little more length to get your weight a little more forward or if you have a hi-rise bar you might try a low-rise bar or a flat bar. One other thing, keep your elbows in towards your body, this will help you keep your weight low and forward and will help keep your front wheel down when your climbing steeps. I would try that first before spending any money on parts. Also as much as you want to stand try as hard as you can to stay seated, you’ll be a better technical and smoother rider in the long run. Standing helps in cetain cercumstances like when you need to get up and over something standing will allow you to shift your weight forward to help you pull yourself up and over an obstical but if your standing and mashing the pedals trying to get over something or in a technical section your going to put to much strain on your drivetrain and there’s a good chance your going to bend your cogs in the back or break your chain. Oh and as for single speeds, ya they’re fun and sure you don’t need to shift but they are not that practical for most trail riding. Sure Travis Brown rides one but he doesn’t race on one all the time, why? because not all trails are the same and sometimes you do need lower gears and sure you can gear your single speed to climb but when you get to a flat your going to spin your but off and not go anywhere and vise versa you can gear it to go well on the flats but when you get to a steep or technical section your going to have to get off and walk up. What’s the point? That doesn’t make you a better rider. The more you ride a trail the better you’ll get. If you get to a section and you can’t make it try it again but try and looking at it a different way and try a different line thru the same section you’ll eventually get it. Your friend also gave you a great pointer, if your on a trail you’ve never riddin before and you come up to a section with a blind spot or corner and you don’t know what’s on the other side slow down a little down shift into the middle ring up front and 1st (granny gear) in back, this will allow you to climb most hills and obsticals and if it doesn’t then you’ll know better the second time you ride the trail 😃. Well I hope this helps you a little and I also hope I didn’t make the single speed guys mad but I do live in Colorado and a lot of the trails I ride on are impractical for a single speed. Have fun and keep on biking!!!