The leaves turning black or brown around the edges indicates that the plant is stressed. Pots tend to dry out quickly and I think perhaps it is getting too much heat being right next to a slider, it may be getting a bit scorched and be very thirsty.
My recommendations would be to move the pot from the position it is in, It does best in a sunny location but being near a window or a slider may be too much direct light and too warm. Increase the watering but ensure the pot has good drainage and that it is not sitting in a saucer of water.
If all else fails, re-plant in the garden outside.
Good Luck, lemon Verbeena is an amazing plant.
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