'97 Mercury Sable won't shift from 1st to 2nd gear

So I noticed recently in my 1997 Mercury Sable that after the car has been running for a while, maybe 15-20 miles or so that when I am at a complete stop and start to accelerate, when shifting out of 1st to 2nd instead of normally shifting between 2,000 and 3,000 RPM's it will not shift until almost 5,000 RPM's and it is a much harder shift which I know is not good to do to the transmission. If I notice it is doing this, if I let off the gas once it creeps up around 4,000 RPM's then eventually it will shift normal into 2nd. It does not do this for any other gears regardless of how long I'm driving and I've also noticed that if I am rolling at all (coming to a stop, etc.) And then try to accelerate it doesn't do this when it's still moving. After I shut the car off for a while and then try again it repeats the process of being fine until a few miles, does shifting problem from 1st to 2nd. I also notice if I accidentally allow it to shift around 5,000 RPM's too many times (for example if I'm driving a longer distance and driving it more after it has the shifting problems) then my "O/D OFF" light starts to blink off and on. Can someone please explain to me what's going on? I'm an 18 yr old girl this is my first car and when I took it to get looked at they said they would have to take the car apart to find what's wrong, maybe a solenoid or something and it would cost a lot so I'm trying to avoid that if possible.