I'm in a similar situation myself. I finally made the decision to transition about 2 months ago as I was just turning 26. I'm not on HRT yet either but hope to be by summertime. Being able to pass before I come out, especially before going full-time, is extremely important to me and has been since I came to terms with being transgender. As some of the other ladies have said, patience is definitely the key and I definitely have my moments where I feel like I could pull it off, and others where I think the cause is entirely hopeless. So far, what has helped me the most has been taking steps toward transition.
How often do you dress/present femme right now? It doesn't necessarily have to be out & about, either. As I came closer to admitting this to myself, I started experimenting more and more with clothing, makeup, & a wig at home. I very quickly went to wearing exclusively women's clothing when I would get home from work or during the weekends. Once you put yourself together, even if it's just a preliminary version, it can really help with confidence. In an afternoon I went from thinking passing would be impossible to being awestruck at the gorgeous woman looking back at me in the mirror! Even after that, it was still hard to see myself as passable until I saw the look in my wife's eye the first time she saw me presenting as 'Libbey.' I couldn't believe how validated I felt the first time I saw my therapist presentin, either. She walked right past me in the lobby, smiled & nodded at me, then we t right up to one of her colleagues and asked if a time slot had been double booked because her 7:00 appointment wasn't there yet and there were only two clients in the lobby and neither was hers. Then, she did a double take and realized it was me with a stupid grin on my face, staring back at her as she realized she had not only gendered me completely female, but hadn't even recognized me at all!
For me, the hardest part of the last 2 months was the waiting. I spent weeks in December just letting myself process my decisions and realizations even though I already had made up my mind to transition. I felt totally stuck during that time and it's gotten much better since taking steps in the right direction. In the last month the alone, I've continued building a wardrobe, started voice lessons, signed up for laser hair removal, and spoke to my GP to start getting my bearings on the medical side, as well as got on Finasteride for my hair. Even though HRT isn't happening yet, there's a ton you can do to start working towards your transition and passing.
I hope this helps!