slipping into second gear
Assuming you have an automatic transmission check the fluid. If it’s okay take it to a transmission shop. An independent shop is less of a gamble but still no guarantee.
I would add to try changing the fluid and filter yourself, if possible, or to find a shop which will do a “drain and fill”–NOT a flush. If changing the fluid and filter does not help, time for major transmission repairs.
Use only genuine Honda transmission fluid.
There’s many problem that can potentially cause that symptom. As mentioned above the common sense first thing to try is a usually replacing the transmission fluid and filter. If that seems to help but doesnt entirely solve the symptoms, after a hundred miles try it again.
For the transmission to properly shift the fluid pressure has to be really high in order to get the necessary forces acting on the shifting mechanism (clutches usually). Over time the seals wear out, small internal leaks develop , which prevent the pressure from building high enough. It has to be really high, like over 100 psi. CVT designs require even higher pressures. New transmission fluid will contain new seal conditioning chemicals, and that can make a big improvement, at least in the short run.