Hello there! I am 20 years old and my fiancé and I have been together for almost 3 years. I just want to clarify, first, that our relationship is totally emotionally and sexually positive. We are completely comfortable with each other. I have not had sex with any other person in my life and neither has he.
About a year ago I noticed that I had a small tear at the opening of my vagina. It never really healed properly and I still have some pain and irritation from it today, only during the first penetration though. But a few months ago I started having pain during deep thrusting. It's a sharp pain, feels almost like pressure (or maybe gas) in my pelvic region. Extra lube helps sometimes, but not always. I don't really understand why all of a sudden I'm having this pain. Maybe someone has had a similar experience and knows what I can do to help it. Any advice would be awesome! Thank you.

Any break in the skin that doesn't heal should be looked at by a doctor. Any break in the skin should also be kept clean to keep bacteria out, allowed air and dryness, and if I were you, I would have some other kind of sex for now to give it a chance to heal.
My experience is with cellulitis, a bacterial (staph or strep) infection that for me started with a hole in my skin from a bug bite. It spread (bright red rash) over my entire leg, and after intravenous antibiotics and 2 courses of pills the rash and infection were gone. But I am left with aches and pains around the original site - behind knee - which doctors tell me I have to accept. I have cramping, stiffness, and pain. Very arthritis/tendonitis-like.

There's no telling if your situation is even remotely related to infection, but I do hope you see a doctor. You also don't know if the tear is related to the inner pain or totally unrelated. Any skin infection can go into tissues, organs, and vessels. But there are many female pelvic conditions such as fibroids and cysts to consider too.