I also isolate when I am depressed. On the other, if the extent of your relationship is texting 3 times in the last month and a half, if that is it, then it doesn't sound like you have much of a relationship, long distance or not.

It can be painful to accept but it might be better, in terms of meeting your own needs, to move on. Long distance relationships are hard. New relationships with a depressed person are hard. Not communicating is hard on any relationship.

It might be easier on your ego to assume he has withdrawn because of bipolar but the truth may be that he has lost interest.