First, I'll get the "older and wiser" lecturing out of the way.
Don't Rush Into Sex
You are only 15 (as you well know) and have a long life ahead of you. There is no rush to have sex. Do I think 15 is young to be sexually involved?
For me, it was. I waited until I was almost 18. Even then, I'm not sure I was truly ready for the world it propelled me into.
You know that you are young. You know that your entire life is ahead of you.
And you already know, as you wrote, that you know you aren't ready. That should (could have, would have, whatever) have been the end of your concern.
You KNOW you aren't ready. You know that doing something because someone else IS doing it isn't a reason to take that step.
The Question Isn't about Sex
Your question I think was ultimately how do you stay with your boyfriend and both be happy. If I knew the true answer to that, I would bottle the answer and sell it at Walmart. I don't know the answer to that. Honestly, the answer to that question is different to every person, to every couple.
Make Yourself Happy
What I DO know (doesn't seem like much does it?) is that
making yourself happy is a priority. If you are not capable of making YOURSELF happy, then you have no chance of making someone else happy.
Only You Can Do It
If I could have actually listened and learned ONE lesson while I was a teenager, it was that no one else can make me happy. I am the ONLY one with the power to do that.
Sounds pretty good in theory, doesn't it? It's a lot harder to practice.
The theory is that if you look at other people to make you happy, you will always be unsatisfied. If I want a chocolate bar, I have two choices. I can wait for someone else to get it for me (sometimes even expecting them to know I want it without my telling them) or I can get it for myself.
Granted, if someone else did it for me, it would be a very nice thing. However, if they DON'T get it, then I sit there, unhappy, waiting, expecting and getting angry because someone else isn't doing something for me that I could easily do for myself. So I get the chocolate bar myself. The chocolate tastes the same, it fills the same need that I had. And I did it myself.
Happiness Is Like A Chocolate Bar
Happiness isn't as illusive as most people seem to think it is. It's rather like that chocolate bar that I crave. The ultimate goal of FEELING happy is the same feeling, whether someone else hands me that happiness or I hand it to myself. But if I choose to make MY happiness depend on someone else, I could sit there for a long time, with an empty hand.
Be True To Yourself
How do you apply any of what I've said to your relationship? Find what makes YOU happy.
If you are not ready for sex, then having sex would NOT make you happy. It might appease your boyfriend on a physical need, but ultimately it could be more damaging to your relationship than waiting until you are truly ready.
Ultimately, you both have to be true to yourself. And if your relationship is right for both of you, then waiting to have sex won't damage it. It will only strengthen it.