You cannot make him stay if he wants to leave no matter how much you beg and plead with him if his mind is made up to leave then he will do that. One thing you should have never done was go off the Antidepressant that seemed to help you to stay calmer and work harder on your marriage. If you tried numerous things to save your marriage and nothing seems to be working, then your spouse has given up and is ready to walk away from the marriage, even if that is the case, until the divorce papers are signed, you still have a chance to save your marriage. Try not to make the classic mistake of blaming your husband for all that is wrong in the marriage, even if he has made a big mistake, it takes two to make a marriage and it takes two to break a marriage. Sit down with your husband and make sure before he leaves that he understands that you are ready and committed to doing whatever it takes to save your marriage, he may not believe or share your positive outlook but it will give him food for thought.
What you should do is to try and look back over your marriage and find out where things went wrong, write down everything that bothered about your husbands actions. Like his going to chat rooms and commenting on picture and then being gone on weekends. It really sounds like he is going through a midlife crisis. Sometimes all it takes to get this mid life crisis is to have problems with marriage issues. He seems to be going through this especially since you said he goes into chat rooms pretending to be much younger than he really is. That is a classic sign of a midlife crisis wanting to feel younger and finding ways to hang out with younger people.
As we get older we focused on reflecting back on life. If we can look back on our lives and appreciate our accomplishments and our contributions, feel a sense of satisfaction and few regrets, then we will achieve a positive outcome of this crisis a sense of wholeness and completeness but your husband did not find that in his life. He may feel his life has no meaning, or his life has been wasted and therefore he has many regrets, he is experiencing the negative outcome of this crisis which is despair, regret, yearning to be young again, living life the way he wants to without having to answer to anyone. Midlife is a stage of life that has more to do with growth and possibilities rather than reaching a certain age and having a crisis but many men don't see midlife crisis as something positive and they often times look at all of the negatives about the crisis.
If you want to save your marriage then you need to take a timeout and try to figure out what went wrong. If you try to get your spouse back without doing so, you may find yourself doing things that will only make your situation worse. So give yourself and your spouse time to reflect on the relationship while you work out a plan to save your marriage. Try and find out from your husband the problems they have in the relationship. When your husband reveals the issues they have within the relationship do your best to correct them. Sure they will have some changing to do also. But focus on yourself first and make the necessary changes you need to make to fix the issues in your marriage first, also you may want to seek marriage counseling. They can help you both to better communicate and find ways to work on the marriage and also they can tell you if the marriage can be saved or if it's time to walk away.
If you aren't sure about marriage counseling but are willing to consider it, one thing you must keep in mind is that this can be a long process, you're not going to find answers right away it may take some time. You may not like for anyone to know your business, it can be extremely hard to open up to someone you barely even know but it's important to have that neutral opinion from someone on the outside looking in. If you want to save your marriage you could also try and do it on your own. It is very possible to do so if you have a plan and know the right way to go about doing it, there is the possibility that you will make things even worse then before, especially when you have no clue of what to do or say to get them back. If you are going to try and do it on your own then you need to find the root of your problem and then find ways in which you can fix the issues at hand.