First of all do not worry, everyone here is super nice and as helpful as they can be. You are not a freak and you are not alone. It is reasonably common for young women to struggle to reach their first, and may I add it is also pretty common for the ones who can orgasm, to struggle at times too!
I cannot remember having a problem learning how to orgasm. It just took me by surprise one day. However in the last 5/10 years I have gone through periods of time where I just "couldn't" anymore (for weeks or months at a time) It frustrated me and my partners I understand that part.
Anyway I can tell you that one of the main reasons for this happening is being too tense, or thinking too much (Stressing) about it. (Rather than it being caused by a medical condition) I can sense that you already know this and have tried numerous ways to combat it, but nothing has worked so far. So this is difficult to give advice that you probably havent already tried or dismissed. Anyway:
First thing is first: You need to learn about your own body, alone. I know this sounds selfish to your partner, and I am not saying stop having sex or anything of the sorts. I dont know about you but I know that I find myself under more "pressure" to perform when I am with a partner. The thoughts (What if I can't cum...and all thoughts that spin off from that) just add more pressure and tension to the situation and can often just switch you off from even trying.
Try to get some time alone, preferably an hour or two where you wont be disturbed. Ok, now the trick is to stop thinking about what you are feeling. "Does this feel good" "Am I building up" "Oh shouldnt I be close by now" all these kind of thoughts are going to help stop you from being able to relax. I cannot stress enough how big a deal this is. When you start to "go into your head" as I call it, and start having worrying or negative thoughts, its like pouring water on a fire. It just kills it. Pushing the thoughts out of your head and just properly relaxing is easier to do alone.
The next thing to do is to start touching yourself. Try different places, different speeds and pressures and when something feels just "nice" remember it, keep doing it along with other things that feel "nice" Do not even think beyond this, just enjoy the sensations at the time. Your goal at this point is not an orgasm. In fact if you make not having an orgasm your goal, it will also take off some pressure. The goal right now is to do things that feel nice.
Next, find things that stimulate your thoughts (Not the bad thoughts) Do you enjoy laying back with your eyes closed and fantasising in your head? Do you enjoy reading some erotic stories? Watching some porn? Sexting your boyfriend....etc etc. Once you have discovered what titilates you, and what "feels nice" from the touching. You can combine the two. It probably wont happen the first time, the second time etc. Its a learning curve but remember the goal now is not to cum, just do do something that feels good.
When things start to feel good then comes the final and maybe the hardest step, letting the feelings come. Although building up to an orgasm is "kind of" like how you described (This pressure building) It also sounded a lot like maybe you tense up at this point, trying to hard to orgasm, or unconsciously trying to dull the sensation because it makes you nervous about losing control, both of these will cause your body to become reallyyyy tense. Now I know that my muscles start to rhythmically tense and relax as I build up, if you are CONSTANTLY highly tense all over and not breathing etc then you are probably too tense. This will also make it harder to orgasm. So my suggestion; As the feelings get nicer or just start feeling different, firstly, no thinking. Nope none! Dont think about why, or when, or what does this mean. Continue to think (or watch) or enjoy your fantasies and dont even think about orgasm. Dont start trying to push for it, and if you feel your body start to tense because you are nervous about losing control remember this, you are not trying to orgasm, and even if you do orgasm you are not "out of control"
Really an orgasm feels intense, but in a nice way, it lasts for about 10 seconds and you can still think clearly and rationally, you can still move every part of your body that you wish to move at any time, I mean, if, mid orgasm, someone burst in to your room, you would have perfect control, you would grab the quilt, pull it over yourself and be able to control your breathing and movements....I guess what I am trying to say is, you wont lose control of yourself at all. (If you have only ever seen women have "orgasms" in porn, it can seem scarier than it really is!! They are so over acting!)
Anyway to sum up, most women who struggle, struggle because of not relaxing (even if they think they are) practise alone, practise as much as you can WITHOUT PRESSURE. If negative thoughts wont go, stop and try later. Try to push everything out of your head other than stimulating thoughts. (That is important) Dont focus on your body. Focus on what it feels like specifically between your legs, when feelings build, dont change what you are doing, dont go "into your head" and over think those feelings just enjoy them, think of it as an experiment, nothing can hurt you, none of the feelings will make you lose control, you will not pee or explode or anything but if thoughts like this are on your mind, minimise them beforehand by doing things like masturbating in the bath or with a towel down. Try different sensations. If no feelings build just relax and end your session and try later.
Can I tell you something? The first time I ever orgasmed I didnt even know what one was, not only that but I cannot remember the touching feeling "good" I cannot remember a build up. I was just sat in my room watching tv and playing, not even aware of good or bad feelings, I was engrossed in the tv show! lol Then BAM! (What was THAT!) So I guess what I am trying to say is, dont think, dont even try to feel (only feel the good things)
Ok lastly, I dont "think" there are pills or enhancers that can aid orgasm. There are some lotions that can "heighten sensation" which just makes your clitoris feel warm or tingly or cool and tingly and can make women orgasm faster or more intensely. However I dont know if they will help here. Anyway heres a good page of products to look at if you are interested in these:
Have you ever tried a vibrator? I would suggest trying a vibrator before you tried the lotions listed above. They are "more likely" to help. I really find that when I am going through periods of time when I cant orgasm because of over thinking things, that a vibrator can push me to an orgasm easier than my hands. However they are a strange sensation to get used to at first, so start small (Maybe a bullet) and again, experiment :D
I hope something here has helped. I really hope you get your orgasm soon!