Is it possible to buy this anti depressant over the counter here?

That brand isn't, but other brands of nortriptyline are: Cetavol, Nortyline, Ortrip.

It's Thai FDA category D (dangerous drug). No prescription required, but a first class pharmacist must be on duty. In practice this means hospital pharmacies and larger pharmacies only.

This is a pregnancy category, right?

Anyway, I am not sure what you mean that this brand isn't? Meaning I can't get in over the counter or in Thailand because I got this prescribed at hospital in Bangkok to regulate my breathing problems and it is helping. The pills were pretty cheap even though I got them at the expensive private hospital, 60 baht for 10 pills.

No, "D" is not a pregnancy category. It simply means that in Thailand the sale of the drug is restricted, so it won't (shouldn't) be available in pharmacies without a first grade pharmacist present at all times. Full description at http://www.mims.com/Thailand/Viewer/Html/PoisonCls.htm

It's possible the hospital imported the drug brand itself, rather than going through a distributor. It's not generally available in Thailand.

Any of the alternative brands I mentioned contains the chemically identical active ingredient and will do the same job for you.