When driving over 65 MPH the transmission disengages?

I got this car a couple years ago and didn't have a lot of information or history to go off.
Previously the shifter cable came loose, and I had one mechanic tell me it was fine, and one told me it needed to be replaced. I couldn't afford a new part. So, I had it "adjusted", he said he used a washer and made it work. I apologize, I don't know more.
Now it's been working fine.
I'm currently on a road trip and on my first day while on the highway I had trouble.
Halfway through my venture, I stopped for gas. When I got back on the highway the car wouldn't go over 65MPH. Once it hit 65, it would fall out of gear. I couldn't give it any gas. Everything still on. No bad noises or smells. No indication anything was wrong except that I couldn't gas. I can pop it back into 5th and gas it. But not over 65 MPH. So, I drove 60 into Tuscan. (These are the same "feels" and symptoms as when the shifter cable came loose - minus the fact that the shifter itself is still feeling tight, not wiggly)
It also can give a small pull feeling, and maybe a small shudder coming out of 1st sometimes. I just noticed it driving in town.
Normally I'd just get to a mechanic, get it resolved and be on my way.
But I'm on my way to a job. And I'm running on fumes financially. And I'm in an unfamiliar town where I don't know a trustworthy mechanic. I've been taken advantage of on a trip before and lost a car because of it.
I don't know if I can make it another 1,600 miles to my destination. Or if I should go the 400 miles back home and accept defeat.
Is it likely, with the age and mileage of the car, that my transmission is going out?
Could it be that the shifter cable is the culprit and needs adjusting or replacing?
I guess, how long would the car last showing these symptoms without intervention? I just need it to get me there. But I don't want to be stuck somewhere between towns with a car that won't move.
I feel like I need some dad advice. (I don't have one, please help.)