I agree with Eiri, see a sex therapist, or a marriage and family therapist if you are not quite comfortable with a sex therapist. It's ok to have a low sex drive, but when you say that you feel nervous or scared that is not something you want to live with. Some suggestions are to help take the pressure off of sex, try things to make it more fun (games, toys, if you are comfortable) and be prepared to spend a lot of time during sex so you are comfortable with the whole process. Go slowly and try to enjoy each other's bodies (I know it is weird when you have been with someone for a long time but it helps) and don't put too much pressure on orgasm. Just try to enjoy yourself. Talk to your partner about it and spend time connecting emotionally as well- this often helps. Get dressed up and go on a date!
However, (and I said this somewhere else too) an abnormally high or low sex drive can be a symptom of many physical or mental disorders. If you are having any other symptoms, like changes in energy level, changes in mood, changes in weight, or anything, see a doctor. It could be many things, depression, hypothyroidism, hormonal abnormalitites etc. If you think this might be possible, see a doctor!!
If that's not the case, then just know that sex should be fun, not a chore. It tends to get that way when you've been with someone for a long time, but it doesn't have to be. Like I said, a marriage and family therapist can help, or just trying to spice it up yourself. I hope this helps.
