I would make myself eat every 2.5-3.5 hours and include big portions of healthy fats. Snack on nuts, avocados, eat salads with heavy dose of extra virgin olive oils for dressing (mix with apple cider vinegar.) Large portions of complex carbs before your workout is a must. Sweet potatoes, brown rice and oatmeal has to be included for your goal to weight gain.
Coconut or almond milk, dairy-free sorbets instead of ice cream, vegan cheese products will pack on calories. You have to make up your mind you will eat 4-6 meals per day and stick to it if you want to add weight.
Also you're in need of adding more iron when you workout. Add either more reps or more weight each workout so you will grow. The more you eat and workout with heavier weight the more pounds of muscle will appear. It doesn't happen in weeks. It's a lifestyle measured in years and decades not weeks and months. Get it?
Have you tried any of the commercial ultra purified and lactose free whey protein or mass gainer powders? You can mix them in water?
Don't kill yourself when you do cardio because you will cause body to release cortisol which is catabolic. So your body would be burning muscle instead of fat and also makes you retain water.
P.S. Grow thick skin your first year at this forum, no one wishes you harm but guys can throw down on posters that have insufficient foundation here. Don't quit tnation, just understand more than half of them wish they had your thin body type. Why? Because it's easier to have your problem and build with the information they have then to work 3X harder to fight their dick-doo disease.
What's dick-doo? Their gut sticks out further than their dick doo. See, you're already ahead of half of them. Only opinion that matters here is yours. So don't sell yourself short ;!