1. I think you should try to let go. You can’t force someone to be with you if they don’t want to, and it’s unhealthy to hold on to something that isn’t yours anymore. You deserve to be happy and you need to let go to do that.
2. “I love you” can be for anyone. The definition of it is a million different kinds of love. You can say it to your mother, your best friend, your cat, etc and it’ll be genuine. However, “I’m in love with you” is used to describe a very strong romantic feeling. You only use it to describe someone you are very serious with, someone you can see a future with. Generally, in a relationship, you would say “I love you” before you say “I’m in love with you.”
3. Well I don’t entirely agree with that, but okay.