Here is a summary of the roles of hormones in pregnancy:

It all starts with progesterone and estrogen that accompanies us since before pregnancy. The level of these hormones rises before ovulation to prepare a "good substrate" for fertilization (there is an opinion that progesterone even helps the sperm cells to reach the egg, and serves as a "motion guide" for them).
After successful fertilization, progesterone and estrogen continue their important roles in pregnancy, such as: thickening of the endometrium, development of the fetus and protection of the fetus, and finally are also responsible for the production of breast milk before birth.
And along with all the good that hormones do, the presence of high levels of them in the body has a very long list of side effects, one of which is sharp changes in mood, and we have absolutely no ability to prevent it.
In the last trimester, in some women, there was an increase in the cortisol hormone (the "stress" hormone). This may be due to the fears and excitement that increase in preparation for the upcoming birth.
So that your fetus can grow and develop in the womb (and eventually come out of it) the pelvis flexes and expands. The main ones responsible for this are progesterone and relaxin, which cause the cartilage connecting the pelvic bones (symphysis pubis) to flex.
The Beta HCG we've all heard of (or the "beta" for short) joins already in the first days after conception and is produced by the fetus (!). Its function is to encourage the "corpus luteum" (the follicle from which the egg burst) to continue to exist and secrete (it too) the hormones progesterone and estrogen.

 
Why are hormones related to mood changes in pregnancy?

In fact, every cell in our body is affected by hormones. And they are responsible, among other things, for: normal activity of the digestive system, the bones, the function of the joints, the skin, the quality of hair and nails, the sexual drive, fertility, pregnancy, memory, our ability to adapt or tolerate cold or heat, and also for our emotions and our ability to regulate them.
As there are changes in their compositions/levels, the risks of "disruptions" in the emotional balance increase, and this is actually what actually happens during pregnancy.
So first, here is a detailed explanation of why you can't (or should) calm down your hormones.

 
How to deal with mood swings during pregnancy?

Accept your sensitivity and emotionality with love, even humor. You are perfectly fine! Every mother who has been pregnant has been there and this is the path we go through to produce life.
"Padded" your environment with people who love you the most, who will always contain you, even when you are not at your best.