You do need someone to go with you. Exit International sometimes offer chaperones for a price. Travel expenses and costs etc.
When contacting someone like dignitas you have to do a fair bit of paperwork and provide medical records. You'll probably need legal representation but I'm not certain.
It can move quickly when you apply but you have to pay some money upfront to get the initial assessment done. It can all happen within 6 months and I've read accounts of it happening significantly faster. I think it may have been as quickly as six weeks if I remember correctly. That was probably somebody with all their ducks in a row though.
If you have mental health issues and they're your reason for wanting to go then you're likely to have an uphill challenge there. Some countries and service providers are more inclined to accept those reasons but generally its not a reason that's accepted.