I think the first thing to do is try to let go of the notion that life is a progression where things have to get consistently better and better. Life is about ups and downs, sometimes more of a spiral than one linear movement.
The notion of constant progression and being ahead or behind at certain life stages is perpetuated by advertising and marketing teams to encourage buying, spending, competing with each other which leads to more buying and spending.
You sound as though you are exhausted and feeling ground down by life at the moment and it's very hard to see a way out, let alone have the energy for it.
Also, you mentioned your Mum having MS - I'm sure the worry about that is also affecting your emotional energy.
When I've felt like this I find the answer is in small, consistent steps. Pick a couple of areas - or just one to start with - and every day do something small to make it a little better.
The way this works is not only do you start to make actual changes, your brain gets a little dopamine hit every time you complete a goal no matter how tiny. (It's why gaming is so addictive)
So for you I'd start with:
Your asthma: every day take one action which improves your health in this area. Extra dust cleaning, researching a good air filter for your bedroom, replacing old pillows, meditation with breath training etc, push for the latest treatments at your doctors. I have asthma and when you can't breathe properly everything feels so much harder.
Making the flat less grotty on the inside: You can't change your location right now so do things to make the inside space more of an oasis so that your body and brain feel nurtured while you are inside. Declutter, decorate, find the colours and textures which soothe you etc. Move all the furniture around to get a 'new' feeling.
Committing to at least 30 minutes outside time in the daylight once a day.
Also as money is tight but you still need stuff to look forward to, see if there are places you could do as day trips or weekend trips where you could rent a bike and do the activity you love.
Take the pressure off yourself about not having everything you thought you'd have right now - make some small consistent steps forward. When you feel your mindset start to shift and you can feel a little more confident and positive about the future then is the time to sit down with your partner and get him on board to work as a team towards something better. Again, one small step at a time.