Hello Purpleturtle,
firstly I would like to say that I am not Raneth but Raneth's partner. I am just using her account for this one and only post. I am of a similar age to you (27) and have very recently experienced a similar (but not the same) situation to you. This is going to sound like a little instruction booklet of 1st, 2nd 3rd... feel free to take the advice or leave it. But certainly things get better with time. My partner is always so up and down with moods. One minute it's "I love my new x", next minute "I regret this so much, I hate myself etc". Honestly sometimes as an SO it is easier just to go "Ok" and don't put any emotion onto the situation. Let her figure out those ups and downs because in reality you can't change that yo yo cycle; you are just wasting energy. Certainly take care of her mental health, but don't own it.
Firstly: don't feel guilty. Although your partner's transition is about her it directly effects your life. It effectively makes two people go through a transition not just one. The very nature of being in a relationship with someone almost always means that you form an interconnected bond that requires shared experiences. Her life and decisions directly affect yours, and that's ok, but you both need to be able to recognise that and be ok with it.
Secondly: If you are not already, talk to someone about it. Cry for the changes in the relationship, feel excited for the future, feel angry and vent... but not at your partner. She is vunerable and really doesn't need that. Take all that emotion and tell it to someone else. If you feel so angry that you need to smash something then do that too. It is ok to have feelings, just deal with them appropriately.
Thirdly: If you or she has family and friends ask them for help. Say "hey mum/ dad/ sibling etc" can you do this load of washing for me, make a meal and bring it over etc. If you don't have that kind of support then talk to your partner about how you can help her to help herself and you. How you can "manage" the daily life tasks together. This includes things like asking her to dialate somewhere where she won't wake you up!
This isn't a permanent situation and things really do get better with time. I was a mess, an absolute right-off after my partner had ffs. I cried everyday for a week (and on and off for a month), went with no regular sleep for 2 weeks as I fed, medicated, wiped and cleaned my partner. Before this I looked after almost everything for her as well (cooked, cleaned, worked, help her organise ffs and so much more!) so I was totally physically, emotionally and mentally exhausted (hence the crying).
But honestly I can say now that things have gotten a lot better. My partner is very understanding and we talk about things all the time. Eventually I gave myself a holiday that was free from her and her problems in order to recharge and that helped.
Think about what you and your partner could do to make this situation easier for you and talk about it. Ask for help if you need it. And don't feel guilty as she probably wouldn't be there, where she is today, without you.
Please feel free to PM me if you need to talk.
All the best