Sex always hurts

I would really appreciate it if someone found the time to answer me because I really don't know who to turn to with this.

I'm 24 and I've been having sex with my girlfriend for over 4 years. She was my first and when we did it the first time she tried to penetrate me with a finger but I felt really uncomfortable and told her to stop. We tried again a couple of times after that but I was never able to enjoy it, so eventually we just stopped trying; I had heard before that some lesbians just don't like penetration so I figured I must be one of those and sort of got over it.

My clit area is also extremely sensitive and even with the gentlest touch I almost always experience discomfort and even pain. My girlfriend doesn't have similar issues and she was really surprised when I first told her, so after a while I started downplaying it and pretending it felt good but the truth is that for the most part I'm only able to enjoy sexual touch when her whole hand is cupping me from the outside (when the clit doesn't hurt it feels much more intense but eventually it starts hurting and I'm not able to orgasm if it does, so I sometimes feel like I have to fake it). That's also the only way I'm able to masturbate, and even though I've been trying different things I am never able to come in any other way. I enjoy topping way more than I do bottoming (although it might be so also because it feels so uncomfortable) so I never really think about this except during sex and then it's over and I just bury the problem somewhere in my mind and leave it alone, but in the past six months or so my sex drive has significantly increased (it's always been pretty high but never quite like this before) and this has started to get harder and harder to ignore.

Then last week I went to a spa and got my period on the same day so I tried to wear a tampon (I got the smallest one I could find, smaller than a finger). I had never even tried one before because even the idea freaks me out but I had no choice so I thought 'if I have to I'll just get over myself and do it'. What happened though is that I inserted it completely and almost fainted, I couldn't breathe, I couldn't see and I started having stomach pains and feeling nauseous. I didn't want to give up so I tried to push it in fruther anyway (it was all in but it had to go up higher) and the thing didn't move in the slightest; at first I did it gently, then I tried pretty hard and still nothing happened. I eventually had to take it out to avoid fainting. I'm not exhagerating, my pain threshold is really not low, - I have tattos, piercings- it was one of the worst things I've ever experienced.

Up to that point I had always believed my issues were all psychological but now I wonder if they may be physical. Furthermore, I really have no reason to have all these issues with penetration and my genitals in general; I've never been sexually abused in any way and I've always had a very open and healthy relationship with sex (also never really had any issues with being a lesbian or my sexuality, either).
So I just don't get it and I'm quite worried about this. If anyone has had a similar experience or just has any inshight at all I would love to read what you think about this. Sorry the post was so long but I still feel like I've omitted parts of the story anyway, I couldn't make it any shorter than this.