shifter pops out of gear

At first I noticed the gears were shallow anyways, but I figured it was just me being used to driving my Honda which is also a Manual. Then right before my 1000 mile drive to California I noticed that every time I hit 4th gear it would hesitate after a few min of being in 4th no matter how hard or soft I placed it in gear it pops/grinds it's way back to neutral. Then it would not accelerate in 5th gear, like all of the power is gone. I continued to drive thinking maybe I just didn't double check my surroundings of the gearshift or had an off day. After multiple times daily and over 1000 miles of having to transfer from 4th to 5th all the time due to changing speed, and gear popping alomg with almost getting me into an accident I have a video of this happening but when I take it in they always say nothing is wrong, they can not get it to "pop out" for them. My gears are grinding now when I shift in any gear, (still only popping out of 4th) feeling like I'm making it work harder than it has ever done. I have not even made a payment on it yet and it has 2000 miles on it now (due to a trip). I purchased it with 5 miles total on it. The problem is gradually getting worse and spreading but, so I'm told.....because I'm a girl maybe I need to learn how to not grind my gears or drive a stick... Let me tell you, I have driven a stick for 15 years and I had my alternate car last 200,000 miles on one clutch so I think I am aware of how to treat a Manuel transmission. The dealer won't do much more and I'm about to reach criteria for the lemon law! I just want someone to listen and help me get this fixed.