I see multiple issues here.

First off, in Texas, it is either too hot, too cold, too rainy, or too windy to ride 99% of the time. So if you're going to do any riding, you just have to get out and do it anyway. Riding fast on roads will get you more wind-in-the-face than off-road riding, which helps in the summer.

That trike sounds like it is heavy, and geared too high for the rider and the conditions. Something needs to change. You may be able to drop the gearing down some. Check with the seller of the trike or a local bike shop to see about that. It won't make a vast difference, but will help some if you drop the gearing to maybe 3/4 of what it is now.

Consider riding on easier courses, and riding by yourself where you're not pressured to keep up with other people. if you're set on riding with those same people on those same courses, consider getting a regular mountain bike. I've not used an off-road trike, but there may be reasons they're not very popular.

If you ride more, and either adjust the gearing or the hills where you can spin on up them, that should take care of the knee problems. On a regular bike, when the going gets tough, you can stand on the pedals, which helps, and generally have lower gearing on the crank, which helps.

Here in the Dallas area, the "mountain bike" trails are basically flat, with an occasional whoop-di-do to keep people awake. It sounds like you might be down in the hill country or something. Anyway, look around some, see if you can't find a flatter course like we have around here. Or just ride on paved bike trails or roads.