If you are finding and MTB uncomfortable, you can do a lot to make it fit you. MTB riding positions vary from very low and racy to quite upright.
You can change the stem height/length and bars style to suit. Look at different brands and models, and measure one with the shortest top tube (shown in geometry charts).
Most MTBs now "feature" larger wheels(27." and 29", which, for smaller and less strong rider makes the bike more of a handfull to manage than the old 26" standard.
Suspension forks are found on all MTBs, so keep an eye on weight.