Giving up a baby for adoption that you yourself know you are not ready to take care of and accept the responsibility of raising is one of the kindest and most loving gifts you could possibly give that baby. I know it must have been one of the hardest decisions you ever made, but I am sure as time goes on you will be able to see that it was the only choice you could make that would benefit you and the baby the most.
Don't beat yourself up. Just forgive yourself and go on. Ask God to forgive you for having sex without being married to that man and plan not to make that mistake again. There is nothing wrong with learning from your mistakes. I like to read the article in the local newspaper written by a former high school principal and he always says that having a sexual relationship with a male is the beginning of the end of that relationship, not the beginning like the people "think" it will be when they decide to be sexually active together. Not to mention the risks of all the horrible sexually transmitted diseases you can get nowadays.
I had a daughter who kept her three kids by three different men and she was not loving or responsible and we ended up raising the two older ones for 10 1/2 years. It was very hard on the kids because the one had a dad who was alive and wanted to be part of its life, and the younger one had a deceased dad and could not spend time with him because he had already died when she was a very young infant. They grew up thinking they wanted a relationship with their mom but then when she matured a little and got them back legally, they found out that Mom was not so great after all. She loves them but does not know how to show them she loves them.
One day perhaps the boy you gave up for adoption will meet you and thank you for being so kind to give him up for a better chance at a good life. He may need to know "why" you made the decision to give him up for adoption, but I would just be honest and tell him where your state of mind was at the age when you had him and whatever all the circumstances were that directed you toward adoption, and I think he will come to understand that it was not that you did not love him or did not want him, but that you wanted a better life for him than what you yourself were capable of providing at that time. I think we are all our own worst enemy. The most important thing is to not dwell on the negative and try to learn how to concentrate on all the positive things surrounding this situation. I find that dwelling on negative does nobody any good, and actually makes things seem a whole lot worse than they actually are. If we practice at finding positives, it gets easier and more natural after much practice. If you ask God to help you realize the positives I believe He will. But you also need to look for the positives. I used to be so much more negative than I am and it has taken a while to get more positive. Most people would rather spend time with a positive person than a negative one.
Also, perhaps some of the crying and depression comes from the fact that your body is still trying to get used to the idea of not being pregnant any more. Some bodies take longer than others to the idea of not being pregnant any more. Remember, you were pregnant for nine or ten months (I forget how long now; it's been 29 years since I was pregnant) and that causes your body to produce more of certain hormones and perhaps enzymes. Suddenly that baby is born and your body has to teach itself to stop producing so much of some hormones and more of others. Some people take more time than others, and also some women suffer post-partum depression. It usually goes away in time, but talk to your doctor. Ask him/her if there are any natural ways you can cope with the tears and depression rather than drugs. Maybe if you took a brisk walk in the fresh air (careful not to fall on ice/snow if you live where that is part of winter) it would make you feel so much better. Or do a few exercises in your home and get the blood pumping.
I got off anti-depressants because they make you so numb that you don't "feel" much of anything. I want to cry when something is really sad. I was numb when I was on the drugs and now I don't have to worry about high blood pressure and the other drug I was on made me crave sweets and I don't need to get fat as a barn, so that was not good for me either. I think perhaps some people can't cope without the drugs, but for those of us who can, do the more natural thing. Of course, God wants us to run to Him for everything. I am still learning to do that more, but the more I go to Him, the less I over spend, over eat, over anything.
If you did not feel any pain over this decision of allowing your son to be adopted out, you would not be human. We were created with emotions for a reason. Allow yourself to be human and give yourself permission to feel sad about the situation. I have found that when I say out loud how I feel and give myself permission to feel it, it goes away so much sooner. One therapist told me once that my emotions would always win, and if I did not learn how to deal with them in a more appropriate way, they would come out some other way that may not be so socially acceptable.