Hey. The fact that you have had no trouble with orgasms in the past could mean that this is not a physical problem. The more you think about it, the more anxious you become. So you and your girlfriend have to create a situation where you are completely relaxed. Your goal should not be ejaculation; it should be enjoyment of the experience. In fact, the goal of any sex between two people is giving the partner as much pleasure as possible. Forget about your own orgasm. You could start by giving each other a slow and intimate body massage, then play it from there. Don't expect immediate results; the idea is to move towards immersing yourself in the experience of sex so that you forget your own anxieties.
There's little point trying to work out why
you are having difficulties. Don't concentrate on the problem -- think about enjoying your relationship and each other's body. Make sure that the circumstances in which you are having sex are as comfortable and trouble-free as possible (for example, privacy, contraception, no time constraints, etc).
I think that, in a year's time, you will have left behind today's concerns about not reaching orgasm in the way you have described here. My best wishes.
One last thing. I have noticed the head of the epididimus that connects to my testicle is a hardish lump on one testicle. I've read places that this can be normal and have been to embarrassed to get it looked at.
If you are seriously concerned about this, or if the lump changes in any way, you must seek a professional medical opinion. Do not rely on what you have read on the web. Your physical well-being is too important to be ignored.