This is my first question on the Interpersonal Skills StackExchange. I have browsed through a number of earlier questions regarding identifying and approaching depression but I have not quite found what I was looking for. In accordance with the Help Center rules I will try to make this more about the interpersonal skills than about the case at hand but I believe some of the backstory might be relevant in an attempt to identify what might lie beyond. Please feel free to point out if/how the question can be improved or if you think it is lacking some specific details.
The Greater Backstory
I've been in a relationship with my significant other for a little more than 12 years now, they are in their late twenties, I'm in the mid thirties. We've been living together in our rented apartment for about half that time. It has not always been easy and there has been a significant breach of trust on their behalf on several occasions about two years ago. I do not believe there has been since and we have recovered from this. We usually get along quite well.
The Current Situation
About two years back, I finished my PhD, moved into industry and am rather successful at it and happy with the job I am doing. They have finished their bachelor's degree quite awhile ago and started the master's at a German university in our vicinity. While they are still technically enrolled they have expressed the intent to not finish and have ceased all coursework since for a variety of reasons. They are not actively looking for a job inside or outside field and would rather prefer to work on a self-employed basis. I am happy to support this but while they get the occasional job/contract I observe a clear lack of effort required to really kickstart their business. Right now, this is going nowhere.
They rather spend most of their free time, of which they have a lot, doing the things they like which includes what I would regard (they wouldn't) as an excessive amount of social media and contact with online acquaintances.
The Actual Problem
Talking about serious and important things has always been difficult with them from the beginning but things are at a whole new level recently. This includes distancing them from close friends who really care about them (because they might raise some uncomfortable questions/topics such as their studying or business progress) and spending a lot more time with loose acquaintances with whom they only speak jovially.
I would wish to discuss our future with them; like moving to a bigger apartment, improving the quality of our relationship and communication skills or, maybe, marriage. I would also like to encourage, motivate and help them to spend more time on and move forward with their business.
I am desperately trying to have this conversation for about four months now and it is going nowhere, to the extent that they would completely ignore me and any approach to bring up the topic. I have tried in person and I have tried in writing (when we routinely spent some time apart due to my business trips). This can include anything from simply dismissing my message text to them locking themselves up in the bathroom to avoid confrontation if I push too hard.
I have tried to let them know that I want to be patient if this is hard for them and that they can bring up any of the topics at any time. This has not happened. I have tried to let them know that this behavior is deeply hurting me, that it is aggravating my insomnia which consequently affects my time at work. They have neither reacted nor have they expressed any signs of remorse. I have tried to let them know that this cannot go on forever and that I have contemplated ending the relationship or at least moving out of the apartment for now. They are aware that I have started to look at some single apartments as I have been very open about all the steps I take.
How to Approach
At first, I thought they were simply trying to break up without being the active party and I have, on occasion expressed that, although I love them, this might be an option if we cannot improve things. I do, however, no longer believe this is the case. We get along well if we're not trying to be serious and they do make joint plans for the medium-term future. No life-changing decisions, obviously, but things like holidays or concerts. While not being able to give much of it back, they are also currently looking for a lot of affection from my side (I have a hard time with this though, because of constantly hurt feelings).
I tend to believe I might be happier if just walking away from this relationship and if their acts are deliberate this would probably be the only option for me. However, I truly care about them and I do not wish to aggravate a situation that is already difficult for them.
At the moment, I honestly think they cannot talk, rather than they do not want to talk and I believe there might be depression at the root of this.
Hence, my questions is: How to discuss future with disengaged and possibly depressed partner?
My significant other avoids any form of serious conversation about important topics for our future and currently leads a life of suppressing everything they do not want to think or talk about. This has been going on for months now. Hence, we're neither able to talk about any serious and important topics in our relationship nor about our communication issues.
What is a good way to discuss the future with my disengaged and possibly depressed partner?