What you are going through is a grieving process, and it's hard.
Because your partner has not offered any reasonable explanation for his behaviour, you have nothing to move on with, so you are drifting, and it is a seriously painful situation to endure.
It sounds - and obviously I am simply guessing here - as though he has just got cold feet at the thought of comitment and a lifetime with one person. Marriage is wonderful and scary at the same time, and he won;t be the first person to run away rather than talk over his anxieties with you.
Friends are sadly little help at times like this - they have a personl stake depending on whether they are friends with you / him / both, and will 'advise' accordingly - none of which really helps.
You need an objective voice to bounce your thoughts off - someone with no personal axe to grind, who has the time to listen, and to be there through this awkward time.
You are very welcome to contact me to chat if you wish -
[email protected] - but no pressure, if you prefer not to, then i completely understand, but the offer is there.
Hang on - you will get past this, although it doesn't feel possible now. Promise.