Good morning, Emerald24!
By all means, talk to OH about how you feel. I know it's hard to speak about your internal feelings or doubts, but there's nothing you are describing that even comes close to the anxiety and fear she must have felt in telling you that she is trans. Many trans folks go for years without telling anyone what they feel inside because they believe that even their closest friends and family members will reject them if they know.
The good news for you is that the misgivings you have about your own body, appearance, mannerisms, etc. mostly relate to things that are partly or mostly within your power to change. Your OH is going through the same process, but in her case she's struggling against much tougher headwinds due to the legacy of a masculinizing hormonal history and the behaviors she learned as a child.
You might want to think of this as a joint project. Both of you apparently want to move in the same direction of creating an appearance that you see as more pleasing, attractive, traditionally feminine. So can you approach this as partners, working together in the same effort?
You might also want to talk with a therapist. You're under a LOT of pressure and you're carrying a LOT of emotional stress. You're working, caring for a child, and dealing with the stress of an evolving relationship with OH in addition to your own self-doubts. A good therapist can help you organize your thoughts and efforts in a way that can help you make the changes you want to make.
Just to put things in perspective, from the point of view of the folks here at Susan's, you are ABSOLUTELY TERRIFIC! OH came out to you - probably fearing for the worst. But you have stood by her and supported her and clearly made great efforts to understand what she is going through. She is SO LUCKY to have YOU in her life!