Hello mummy, congratulations!
1. I have no experience with flat/inverted/short nipples, but according to my mother in law who had similar experience she said she had to pump first to get the nipple out before latching on. I had sore & chapped nipples during the first few days because had very little milk. You can try to get those nipple cream and apply after feed to ease the pain. No-cost method is to squeeze out some breastmilk and apply it over your nipple and areola area, then air for 10-15minutes.
2. To unlatch your baby, you can insert your pinky finger through the corner of his mouth in between his gums. This way you can unlatch without hurting you or the baby. I made a big mistake last time by just pulling out and it was very painful! Also, do try to let your baby feed on the other side, even if it's only a few minutes as the beginning. Balancing between both sides also prevents lopsidedness.
3. Women in warmer climates often produce thin milk. The milk will look very diluted as compared to formula milk. This is very normal as babies in warmer climates require more hydration, thus the thin milk is the result of more liquid. Babies in cooler climates will have thicker milk as they need to keep warm.
To ensure that your baby obtains the hind milk, make sure your breast is emptied out before you switch sides. You know your breast is emptied when it feels very soft. Please eat well and drink as much water as you can. It is very normal to feel thirsty throughout the day. Eating well also ensures that your body is producing enough milk.
4. I had the same problem as you because my girl will fall asleep during feeding. If your baby falls asleep and you are quite sure he has not have enough milk, wake him up. Sometimes they cry because they want to be held close to you. Babies love the skin to skin contact with mummy!
Please try not to be so stress! Stress will make your body produce less milk, and this may also be a reason why you need to nurse more frequently. Try to get as much rest as possible. If baby sleeps, take a nap as well. Sleeping will help your "factory" to produce milk! dont't worry, as long as you still have milk, it will be enough for your baby. It's only a matter of latching more often