I don't know much about bicycles, and I have relatively little experience with them. I bike for fun, or to get around the city, not for sport.
Recently I bought a BTWIN Rockrider 340 mountain bike, in size M. My height is 168 cm.
After going out for a ~35 km ride, I found it somewhat uncomfortable and I got a back ache. The handlebar seems too low. It's much more comfortable and relaxing to just touch it with my fingertips instead of holding it properly, as I don't have to bend down that much.
How can I tell whether I got the wrong size bike for my height or whether I simply did not use the correct posture?
In the country where I am currently, exchanging the bike after such a ride is probably not possible, or very difficult, so it is really a last resort to try. While I did try the bike before buying, the store was in the city, on a high floor of a multi-story shopping centre, so there was very limited space and I didn't realize the problem. While people on this site might not think much of this bike, for me it was a fairly pricey one, mostly having had second hand bikes before. Therefore I would really like to make sure that I did not get the wrong size, or otherwise correct the problem somehow.
I got this bike for getting out of the city and going to places inaccessible by public transport, mostly rocky dirt roads.