Background, both of us currently work FT earning similar ish amounts. I earn £28K and he earns around £32K with overtime.
Been together almost six years. The flat we live in is solely in his name a she bought it when we had only been seeing each other for a couple of months. It was always on the radar to add me onto the mortgage but we genuinely never got around to it - not because he didn't want me on it.
So fat forward to now, DP is fed up of doing what he does and has applied to join the Navy as a trainee engineer and he has got in. Great I guess.
He will now earn £15K for three years until he is fully qualified and then will go back up to £35-£40K so this is genuinely temporary.
How is it fair to look at money going forward?! I'm happy to take on the bills but I begrudge paying his mortgage - the property is now in negative equity so I don't think it would be wise or even possible to add me on now?? So as well as his mortgage he is contractually obliged to live in halls at college which is going to cost around £600 a month. His mortgage plus his halls will be almost his entire salary so should I then be giving him some form of... Pocket money?! I don't want him to have nothing but similarly he has chosen to do this so I don't want to leave myself with nothing either, I work hard 
Can anyone see any way of doing this in a fair way to both of us? I am happy to pick up the shortfall and thus was all discussed before applying but I don't want to spend three years completely broke so he can follow his dreams.