Firstly, you've worked so hard to get to this point and you should be really proud. Intellectually, you can do this no problem. I know you've been out of formal education for a little while, but you'll get into the academic swing of things. Is there some light academic preparation you could do leading up to it? I'm not sure if you have log in details for your university yet, but where i went, if you had log in details, you could access (any) lecture slides online. If it's the same with yours, could you perhaps have a look through some of the material? Obviously not to learn it, but to familiarise yourself and try and maybe get yourself into that headspace before it starts. Also, with first years, they ease you in with introduction lectures, and teach you how to write essays etc.
Secondly, whilst your struggles are very real and very debilitating, it's good you have a few extra years of managing your conditions, and an extra level of maturity is great. I know you've been in contact with your support services which is brilliant. When I started, it took months and months for me to be under their radar, so that is really great. Try to be as open and as honest as you can early on about things. Let an academic member of staff such as your tutor know really early on too as they are vital for links between you and the department. My DofS was incredible and helped me immensely, but again it took too long for them to find out.
Thirdly, and i know i don't need to tell you this, but be kind to yourself and take things as easy as you can. Pace yourself with work, societies and social stuff. Don't put pressure on yourself. I think a little bit of planning prior to going is essential here. I would list what you would like out of your first term (try not to think too far ahead at this point), and maybe make a list in order of your priorities. Then think about the minimum that would need to happen for you to feel satisfied that you'd accomplished that. There's plenty of time, but if you rush in too quickly it could go wrong. Perhaps your first term could be spent trying to get back into the academic swing of things, making a couple of friends, and being involved in one society.(obviously these are examples and yours may be different). Remember that you will have a steady stream of work which will need completing, but societies often run termly and it's not uncommon at all for people to dip in and out. This might seem a bit patronising so apologies if it is, but it might also be worth writing a list of 'warning signs' for you, in terms of signs that things are starting to go downhill, and things you could do to help manage this. I know it seems as though it may be obvious to you now, whilst you're in the whirlwind it might be more difficult to see.
Is there anything in particular I can do to help? If you have any questions or anything please let me know.