If normal sex gives pain, it hints that she is tense, and not turned on.
When you say "I thought talking to her might gain her confidence, but she does not feel prepared and I can't seem to change anything about it. What can I do?" it almost sounds like you are saying that now that you have been in a relationship for 16 months, she should be giving you sex.
However, you two are teenagers, and no matter what your bragging guy friends say, not every teenager is having sex, and if she says she is not prepared, she is not prepared! She has the right to want her virginity to go to a man she totally loves and in the context of a very serious relationship (the kind that might end in marriage). It's not just a toss in the hay, to girls.
If she was sexually experienced and having pain, I would say, yes, she should see a gynecologist. But when a girl is 17 and a virgin, if she is having pain at an attempt by a penis to go in, it is an indication she is not lubricated, and the most common reason not to be lubricated is that she is not sexually excited by what is happening. She might be trying, out of a sense of obligation to you, who is complaining about the dullness of your sex life. I am sorry if that is so, because it is no reason to give up one's virginity. Obligation is not a turn-on.
Anyway, despite the longtime nature of your boyfriend-girlfriend relationship, she is not obliged to lose her virginity to you. I am sorry but there is probably no magic phrase you can say to her that will suddenly turn her on. A lot of girls (especially in the teen years) love kissing and stroking and petting but are not excited about the ultimate commitment that losing their virginity represents. She might be one of them. And I might mention that if petting and oral sex are getting boring, it might be that the relationship on the whole is getting limited. Don't let it be that the only thing you two do when together is joyless masturbation or oral. One hallmark of my teenage relationships is that we did sort of run out of things to do that were fun, try hard to think of new ones that aren't centered around sex, and that might heat things up.
If normal sex gives pain, she should consult her gynecologist or urologist.
Hi, how old is your girlfriend? Any female has the right to not want to lose her virginity, and this is especially true if she is young.