#1) I fantasize about being in a relationship with Twilight Sparkle. I can't seem to stop myself. What should I do?~
Answer: While the behaviour is, of course, unusual, at least you realise that something is wrong, which means that you're halfway there to winning the battle. While it's healthy to develop certain bonds with characters from a TV show, there's obviously a point where reality and fantasy should remain separate. That point, it appears, seems to be when you start imagining yourself in a relationship with a pony.
While you're sure to face a degree of ridicule based on your confession, it's important to understand why you feel attracted to Twilight Sparkle. If it's the qualities that her character conveys, such as punctuality, hard-work and intelligence, then those are all fine traits that you perhaps wish to find in a real-life partner. Given the detailed characterisation of the ponies in "Friendship is Magic", it's not beyond the realms of comprehension that people would develop strong feelings for them based on the exploits that they overcome and the behaviours that they demonstrate.
The obvious answer is to try and distance yourself from Twilight Sparkle. Maybe you should make a list of all the things that you dislike about her character? For example, she has a tendency to go crazy over tiny, insignificant things, and she can be a bit of a know-it-all. If you spend a lot of time looking at images of Twilight Sparkle, try looking at some other ponies, or even stuff outside of pony art. Your infatuation may have come from an obsessive adoration for the character stemming from hoarding art and literature of her: change this as soon as you can.
Obviously, your issue goes deeper than I can engage with here, and there's little that one person can say to make you change how you feel. But if you genuinely want to have a relationship with Twilight Sparkle, then the important thing to realise is that you want a relationship in general: it's possible that you want to invest in another person and be there for them, which is never a bad thing. You just need to move away from the pony and find another human being who displays those characteristics. An unrequited love for a pony may not seem too damaging, but it's holding you back from making positive changes to your life.
#2) Other pony artists are more popular than I am. It's not fair!~
Answer: Popularity can be a double-edged sword. While you might like to be popular, there's nothing wrong with remaining relatively unknown, or even having a small but loyal band of followers. People like to think that popularity is something to aspire to, but having thousands of watchers and fans isn't always a good thing. Some people prefer to know all of their watchers by name or icon and have lengthy discussions with them. If you're really popular, you might lose out on that close level of interaction.
While you might not think that it's fair that others are popular within the fandom when you're not, you shouldn't target other people due to their popularity. People work hard to get respect and followers; if someone is popular, you can be sure that they've put a lot of effort into getting there. Sometimes popularity comes from raw talent, but other times it can come from a good idea. As long as you're passionate and willing to get involved in a variety of projects, your name will start to get around and people will begin to recognise you.
Sheer popularity is reserved for the few, and you shouldn't worry if other people are getting more views on their pictures, or music, or anything else than you. While the initial reaction is to get jealous and think that life isn't fair, you're better off avoiding feeling sorry for yourself; pick yourself up and get back out there. By applying yourself, you'll do far better than just sitting back and complaining. Don't spend your time concerning yourself with what others have. Focus on what you have already, and set yourself realistic goals.
#3) I want an alicorn OC but I'm scared people will hate me for it. Should I make one anyway?~
Answer: A lot of people have a problem with pony OCs that are, at least in their minds, poorly designed. You'll find that certain aspects of a design are objective; different people may like what others hate, and vice versa. By all means, having a typical earth, unicorn or pegasus pony OC is reasonable, but some people might like to try to deviate from normalcy.
If you'd like to make an alicorn OC, for example, then you might get a bit of negative attention, but generally people shouldn't bother you too much: if you feel that having a horn and wings will benefit your character, then go ahead and do it. There's something of a tendency among some pony fans to get elitist and snooty about design choices. But you shouldn't base your artistic decisions on what other people want or expect: you're free to design whatever you want, and nobody can stop you.
Alicorn OCs are frowned upon, mainly, because of canonical rules of there being very few of them. In addition, alicorns seem to be royalty, and so if you make an alicorn who is also royalty in some way, you may be accused of making a character that's too overpowered. However, the most that you'll receive if this is the creative direction that you want to take your alicorn OC in will be a few negative comments, which you can easily choose to ignore rather than engage with. 'Alicorn' has become something of a dirty word within the pony fandom, but if you're willing to deal with the stigma associated with it, then go ahead and make one. After all, it's only pony art: if people hate you for it, fuck 'em!