I'm doing great, I design, sell, print, pack and ship all my own line of shirts. I no longer need to work a second job. It is what I do full time. I have my shirts on some popular retailers as well which brings me more business.
BUT, I still have a long way to go, it's never over. I am always trying to learn.
Why do I print my own shirts, hoodies and tanks? Simply because I want to know the quality my customer receives and want to keep it in house. And as my business keeps growing, I will hire people. I cannot expect them to be good at printing or any other aspect of quality if I don't know how to correctly train them.
It also cuts out the middleman and I know exactly what is going on.
Only other advice I can give again is on quantities. I personally know a lot of companies who fail by bringing out too much product at once. You never know what will sell until it is available to buy. Some of my favorite designs that took days to create have sold the worst. Some designs that took 20 minutes to create sell the best. You never know. It makes more sense to release a few at a time, and in small batches. You can always go back and have more made. What do you do if you make 500 shirts and they don't sell? If somebody doesn't want it, it doesn't matter how much you discount it, they won't buy it. Discounting items is a red flag that you are hurting to make a sale. And customers don't like that.
As you release things you see what is working, you keep those designs, and remove the rest and replace them with new ones. After time you build up a catalog, it takes a lot of time and work.
It's a different world than printing for somebody else, but I think it is a rewarding one.
Never count on sales until you have the money in hand. People will promise and promise they will buy something. 1000 people will tell you "Oh I'd definitely buy that!" Forget it, don't even think about it until they actually do. Because most people will not.