Wow. For me its kind of the opposite. I eat too much I think (Around 2200-2500 calories a day) and I don't exercise enough. I'm 5'8 and 137ish. I'm trying to slim down a bit but I can't seem to eat less. I tend to snack a lot throughout the day, especially between breakfast and lunch. I usually eat breakfast at 6:30, a snack at 9:30, lunch at 12:00, second snack at 6:00 and dinner at 8:00 (yeah I eat late). Idk what my ideal weight is but I've been doing pretty terribly recently and I'm definitely one of the bulkiest distance runners so I'm trying to slim down a bit and hopefully I'll get back on track.
For OP, snack a lot more throughout the day. As others mentioned, trail mix is a great snack and has plenty of calories. Another great way to add calories is drinks. Protein shakes, smoothies, anything like that is great.
Good luck.