Hi Sian, and welcome!

Many of us can live in denial for years, trying to suppress our dysphoria so we can concentrate on other priorities. But eventually we reach a crisis point: a point at which our need to transition becomes unbearably strong and we can't keep living the lie for one second longer. It sounds like you may have reached that point.
Here's the hard truth: you're a woman, and you cannot continue struggling through life pretending to be a man. But your current girlfriend doesn't want to be in an intimate relationship with a woman... even whilst she's still presenting as a man. So as much as you might love her, if she's not able to love you for who you are then it's highly unlikely that your relationship will succeed in the long term.
Of course, you could try to suppress it; stuff yourself back into the closet and try to make a go of this relationship. But who are you living your life for? Partners come & go, but you're stuck with yourself 24 hours a day, every day, for the rest of your life. If something about that is making you miserable, then you need to concentrate on fixing that first & foremost... and then the right partner will come along when you're ready. The more she thinks of you as being a woman, the more she's found that she's not OK with being in a relationship with a woman. That would explain why your intimacy has dwindled & why you're currently no more than housemates: she's recognising your femininity & she's not comfortable with being intimate with you. She probably isn't happy with the idea of even sharing a house with another woman (many women would hate that).
I'm afraid it sounds like she's already checked out of your relationship. You
could waste a couple more years in boy mode whilst trying to win her back, but if she's already checked out then she'll probably leave anyway... and where would you be? Just as single, a couple of years older and still desperately needing to transition. Why waste that time?
My advice to you? End your romantic relationship and try to maintain a friendship with her if you can... and start your transition as soon as possible. Transitioning takes
years so the sooner (and younger!) you start, the better your results. You haven't said where in the world you are, but as an example in the UK it usually takes around a year just to get seen by a gender clinic, let alone starting any treatments. I notice you've chosen a Welsh name; it's even worse there than in the rest of the country. But wherever you are, get the ball rolling as soon as possible.
I was married for 18 years - with two kids - before my dysphoria became so unbearable that I had no choice but to transition. I'd managed to distract myself with my career & family for all that time, but like most trans people I hit that crisis point and I started my transition at age 40. When I told my husband that I was going to transition, he rejected me (and our 18-year marriage) because he's not only heterosexual, but also homophobic. And people are entitled to their own sexuality: if he's heterosexual then I have to accept that & I cannot force our relationship to continue. It was very painful at the time, but I don't regret it one bit... because why should I waste my precious time with someone who isn't able to love me for being me?
Why should
you?