I don't think that I will be able to continue being happy in bed if this is how it is forever.
This is an ultimatum, albeit phrased more politely. And that's the problem: it's still an ultimatum because it communicates "change or I leave (eventually)".
Think of it this way: whenever your boss asks you to do something, he adds "or else you're fired". He's not incorrect, refusing to do your work is grounds for firing, but the fact he has to say it every time makes you feel like you're being strong-armed into it.
As a boss, it would be better to simply ask to do a task, without any threats. And only if it gets too far will the boss bring up the alternative options of being fired. Now, a kind boss should still provide a warning, but that's not the same as always reminding them that they could be fired whenever anything is asked of them.
In your case, this means you should try to spice up your sex life without discussing the inability to be happy with the current state of affairs, or breaking up.
Consider the difference between:
- I want us to go eat at the Italian place.
- How about that Italian place?
The latter sounds much more open and inviting, you're discovering a new place together. The former sounds much more like you (singular) already decided where you (plural) should eat.
Now let me use similar words to yours:
I don't think that I can be happy tonight if we eat at the Thai place.
Do you now see how this comes across? Even if you wouldn't particularly have used this phrasing in a conversation with her, it is how you think about it and thus how you subconsciously will approach it. And that will leak through to her, even if only subtly.
Especially regarding sex, you need to stress the togetherness, rather than getting someone to keep up with you. Because they won't.
Don't push the wagon. Simply float an idea and see if she takes to it. Even if you have to float many ideas and she's caught on that you're looking for something, you should still avoid expressing anything regarding unhappiness. For example:
I've just been feeling interested in exploring new things with you.
Compare this to:
I don't think that I will be able to continue being happy in bed if this is how it is forever.
While both are equally accurate (and I'll admit that I've painted your words ruder than they were intended) there is a vast difference in tone and focus.
If she always abjectly refuses, or simply is unable to enjoy a spicier sex life, then you will have to consider your options. I would suggest you first have an open discussion about how you feel, rather than breaking up out of nowhere or talking about how she's not meeting your needs.
If nothing ever changes, and you've tried everything, then maybe it is time to consider breaking up. But the core of the answer here is that there's nothing to gain from prematurely warning someone about the eventual possibility of being unhappy or breaking up at the first sign of possibly not getting your way. It only leads to a feeling of being strongarmed or (politely) pushed into something.