Obviously 500 calories per day is way too low. You know that so I won't go further into it.
First of all, most people eat breakfast. Whether that means getting up a little earlier to eat something (and then either go back to bed or have a bit more of the day to do things!), or eating a late breakfast once you're up at your usual time and then moving "lunch" and "dinner" to a little later (e.g. a meal at 1pm, 5pm and 11pm, rather than just 1pm and 7pm or whatever).
Have bigger meals. Increase your portion sizes – more pasta, more rice, more everything. Make sure you get in some protein, fats, vitamins and minerals as well as carbohydrates. Add more things to your meals to increase their nutritional value. If you feel that you struggle with larger volumes of food, then increase the calories by, for example, adding a load of butter and cheese to your pasta, spreading an extra thick layer of peanut butter on your toast (plus butter, banana, jam, honey etc. as well as peanut butter) (Lidl and Aldi do kg tubs of peanut butter for like £3.99 last time I checked) (heck I eat it by the spoonful straight from the tub lol). Basically put more calorie-dense foods into what you're eating. Plus make your meals all really tasty so you want to eat loads
, and eat everything on your plate.
Snacking isn't necessarily unhealthy – in fact it can be an important component to your day in terms of getting food in. Plus (quiet) library snacks hehe
Also, "healthy" ≠ low calorie.
I have found that getting into a routine of having a snack at xxx time (and again later in the day) can really help to a) make yourself have some more food, and b) actually increase your appetite.
If you don't fancy eating something, how about a drink? Milk, for example, is pretty cheap. Straight up, or make smoothies, shakes, milky teas/coffees etc. You can also stick a load of oats, peanut butter, etc etc into a smoothie without really noticing too much.
Cheapish things that are often on my shopping list – coming from a hungry gal who eats a TON and also has a student budget:
Oats, rice, pasta, some bread – look for the "basics" range e.g. the Sainsbury's Hubbard's Foodstore or Stamford Street Co., often "price matched" to Aldi/Lidl or if you're lucky even cheaper.
Lidl/Aldi kg tub of peanut butter.
Carrots are reasonably cheap, and a few other types fruit and veg depending on season and state of the Cost Of Living Crisis. Also look in the frozen and tinned sections. Dried lentils, chickpeas etc – take a bit of time to cook but worth it.
Cheap yoghurt, cheap jam, cheap chocolate.
Depends what you eat really. Shop around, make the most out of loyalty cards and bonuses/discounts. Yellow stickers.