Well, if she has lied before, and you are not with her most days to see the truth for yourself, then the jealousy thing is a little more understandable. Not good, but understandable.
By the way, how do you know that lots of boys at her school try to talk to her?
How do you know that she says she is not interested?
Have you discussed this?
Perhaps you have been affected by your previous relationships, and so need more reasurance than usual ~ especially if this girl means so much to you. The anxiety won't help ~ so yes, get counselling.
Perhaps you could write to her.
You could explain
~that you really care for her
~and that you know that the issue of your jealousy is doing harm ~and that you are trying to address it.
Remind her
~ that you tend to be anxious
~ you are suffering because of the legacy of untrustworthy women
~ you are hoping to get counselling to help with all this
Explain
~that she is so special to you that you automatically think that other boys will find her special too
~that you are worried that you might lose her ~ especially as she is surrounded by lots of boys at school and you are not even there with her; not protecting her; not sharing time with her; not being part of, or knowing much about, that aspect of her life.
~That you are naturally envious of those who
do share this time with her
~Admit that you can be jealous of the other boys in her life, and apologise for this, but ask her to try to understand
Tell her
~that you will try to curb the jealousy
~that you feel that you should be happy, as long as you are the only special boy in her life
~that because of the past, and your personality, and the fact that you & she spend your days apart, you just need extra reassurance.
Be careful about how you go about this one, but try to ensure that there will be no more lies between you in future.
As they say: If you want to be trusted, tell the truth!
Only say the things that are true and that resonate for you, of course, and, if you can patch this up, try to spend quality time together & try to honestly share stories of your days.
Good luck!