I, too, have a hard time gaining weight. There's a few things that have worked for me in the past. I have had a couple successful ~20 pound bulks, with relatively little cutting (usually I just naturally pop into a loooooong recomp phase).
First is mindset. Do not commit to the eternal bulk. I find when I do this I grow to hate food, even though I love cooking and eating delicious food all the time (just not in ginormous quantities). Instead, pick 3-6 months of your life that you expect to be relatively unstressful. Building muscle is stressful. Eating more than you want to is stressful. Additional stress can get in the way of your gains. If you just pick a few months out of your life to get bigger, it will give you a goal to work towards, instead of a permanent affliction that you bear.
Next is food choice. It goes without saying that you should eat ample protein if you want this weight to be muscle. I will not preach to you on this point, but just gently remind you to enjoy protein throughout the day. Aside from that, you want calorie-dense foods, foods that go down easy, and foods that make you hungry again. Plenty have been mentioned in this thread already. Here are my favorites:
Calorie Dense Foods:
Dried Fruits <---- These are unbelievably dense in calories. Yes it is sugar. Don't worry, it comes with fiber. You're trying to get big, right?
Nuts
Oil/Butter
Chips
Foods That Go Down Easy:
Juice
Milk
Smoothies/Shakes
Foods That Make You Hungry Again
Carbs. Pasta. Rice. Sugar. Oatmeal. You might think that big bowl of oatmeal/pasta is going to keep you full all day because of how it feels in your stomach. This is an illusion of the present moment. If you eat a lot of carbs, you will be persistently hungry. Honestly, in my bulking successes, I focused more on carbs than protein.
It helps to have certain foods you look forward to eating. This is a big reason why I have made a hobby out of cooking. It is far easier to convince me to slam 1500 calories in my mouth in one sitting if they are all delicious.
Do not be afraid to eat these things in quantities that some may consider excessive. People may even criticize your diet as unhealthy. I'm not a dietician or health expert, but I reckon if you eat lots of protein, and keep the junk food (chips, candy, etc.) to a minimum, you will probably be fine unless you have some other health condition that precludes these choices. Also remember that this is a temporary state of being that you are using to obtain a goal, not a life sentence. It is unhealthy to be chronically obese. As far as I understand, it is usually not that unhealthy to be big and strong (you might just naturally recomp and become ripped afterward, anyway).
One final piece that I highly recommend is that you keep track of your weight and calorie intake daily. This might stress you out a little, depending on your personality, but it will actually be your best tool because it will let you know when to eat more (or when you can eat less).
This might feel like a slog at times. Don't be too discouraged. Commit for a period of time and get it done. For 10 pounds, as little as 0.5 pounds/week makes a pretty easy 5-month bulk. I would even go as much as 1 pound/week, just to make sure you're definitely over the line. That would only take 2.5 months.
If you PM me we can chat more. I have some other resources that I've used as well.