I somehow missed this post last night--sorry. Did you get started today?
Yes, I absolutely think you must build up the life beforehand. Make it a great life. Then have things start to be amiss. Little clues that later the reader will think back on. Give him problems that link to the actual events that transpired to put him in the mental hospital. One thought that is interesting is that the same thing is happening again, almost, in his psyche. So he is recreating the events that already transpired that cause another break in his psyche. Maybe let the texts support that--somehow link them in so they are not just a person telling him his current life is false, but INDUCING the break in his psyche, and they are the same person that did it in the past to cause all of it to happen in the first place. Then end is that you give him his perfect life back...then he gets that text again (so it is just going to be a never-ending cycle). Just a thought that you can take and improve on. Then it is an odd past-present-future in ONE reality. (You might have to layer and go back or outline first so you can keep it straight in your mind.)
It is a good story--actually very good. If you don't write it, I will, lol! My husband is also an author and thinks the same. You have a good vocabulary and writing style for your age. Definitely write it.
Look into Memento for the way to deal with the fiance (great movie--one of my all-time favorites). It is quite similar. Don't copy, but be inspired and take it further.
You have a lot of options on how to present the story in terms of tense and voice. You can make the construct happen fast (hour/days) or extend it for weeks/months/years if you choose. You can do both, as well, as the book progresses. You can insert flashbacks or flashforwards, but do so sparingly and to push the plot forward.
Also, look at Shutter Island w/ Leonardo Di Caprio, also a book by Dennis Lehane. It is almost the same construct. Identity w/ John Cusack is very similar.
If you want help or someone to look it over, PM me. You'll need to attach it as a file b/c the length won't go through PM. (I don't know how I can make corrections/comments for you that you can easily see and decide to keep/ignore other than in Word. That is how I do it for authors. Otherwise, it will be difficult for you to see what I am referring to and make the changes.)
Psychological thrillers/horrors are hard to write and keep straight in your mind, but just keep at it. I am still working on books after 12 years. You have a good story that is not often done, so take your time and really let it come to life in your mind, esp. after being inspired by the other plots I mentioned. (Or write a first draft, then look at the other stories to add new elements and layers if you think you'll be overwhelmed--everyone is different in how they like to write. I avoid stories of like while writing to prevent muddling mine, but others need that inspiration.)
