I think your significant other is fortunate to have someone like you.
I'm not married, and have been through several relationships during the past 15 years, and maybe one of my seven girlfriends was able to demonstrate that she understood me.
Of course, I feel like no one understands me except people who have walked in PTSD'land. It's a horrible, isolating world, and for 20 years, I've felt as if I lived on an island of despair.
I can see very clearly how, for you, it must be very challenging to relate with your partner's mental challenges. My family -- mom, dad, bro, sisters -- choose the route of not broaching the subject of my PTSD, and this does none of us any good.
So kudos to you and best wishes in your journey. May things get better, and easier, for both of you.
PS - I like "I'm proud of you" and "I'm here for you" and "I love you." Of course, your response would depend on the situation. Sometimes, I don't want to talk or hear someone grasping for words in an attempt to comfort me. That's way, as a blanket rule, the simple phrases you can always keep in your conversational repertoire.