"Just being there" is really the best advice that can be given to any one in any relationship!
Yet "there" is often a difficult place to pinpoint. It's a simple phrase but "being there" is different depending on the triggers and events and person.
I'm in the same situation as OP but without planning for kids. We are simply taking it day by day though tentatively planning a few future "things".
My SO suffers from severe, sometimes physically debilitating anxiety and depression. A part of it is situational, and there's an external component that can't really, as of right now, be removed.
I sometimes feel so utterly useless, terrified that I'll fail them or hurt them more, but I can't tell them that. Not in that way. I do ask what they need, and I try not to push myself on them when I feel they need space but I try to be there. All of us, that's really all you can do, it's try because ~no one~ gets the instruction book.
I hold them. I hug them. We cuddle a lot. Touch. No end game on the touching. It helps me as well, honestly. Small things do add up. Cultivating grace, acceptance, love, tenderness, patience, awareness. They don't cure but they can help mitigate issues.
When things are rough I try and make the time we spend together (we don't live together) into a safe space. For us both. Because yes, I need them to be there for me as well. This hit me during a bad episode where we both had a pretty wrecked day at the same time. You really -need- to take care of yourself, as an independent person, if you wish to keep taking care of your love, who is also an independent person. And sometimes it's just acknowledging that things can be shitty but they are less shitty with them there.
We can give each other comfort from our demons. It can sometimes be difficult, navigating the reality that someone is your partner and not a fragile child that will shatter. Because they aren't a fragile child. They are strong, and stubborn, and wonderful, and driven, and yet, during episodes my nurturing side goes into full overdrive sometimes and it's difficult to remember.
So sometimes that "i want to help" desire doesn't help... At times all I want to do is protect them and that leads to treating them with kid gloves. That's not a good recipe for a healthy relationship. And then they also want to protect me from the worst parts, from worry and stress and that leads to them treating me with kids gloves.
Like everyone else, I have my own issues that I deal with, intimacy problems, self-worth, depersonalization issues, anxiety, and difficulty with verbal communication and so this makes the two way interactions difficult sometimes.
But when I'm struggling, or having doubts, I've often found simple touch can work wonders.
/ sorry. this actually came out more rambling and much longer than i intended. I guess i needed to get some of this out.