Hi and welcome to the forum, tell your son somebody said happy birthday. My kids are very young, one is three and the other will be a year old in a couple of weeks. My divorce was final in June and I took the split up very hard. I checked into a mental hospital in may and got out and the end of June.
You may not get to see them every day but instead of brooding over it please try something positive. Make them something from your heart to be given to them when you see them again. It is better than buying them something. Remember the positive things from your time with them and except that now you may not be able to see them as much as you want. The time you spend away is not as important as the time you get with them.
If you spend your time without them brooding over not being able to see them when you do you will likely so everwhelmed with joy or anger and neither of these are good in large doses. You could cry if your joy is too high. You could be depressed if your anger is too high. However if your joy is too high it will lead to depression once they leave. If your anger is too high it will keep getting worse after they leave.
Depression only depletes us.
Unresolved problems do not go away, they worsen.
Notice the ironies of life and enjoy them.
Good Luck
turn to your family..:) don't shut anyone out.. they all care about you.