I hav enot read the other responses you received because I do not want them to influence what I am about to say. This is my initial gut reaction to your situation having been through basically the same thing myself. I apologize now if this comes across as harsh or uncaring. It is DEFINITELY not meant that way.
One thing I want you to remember, if you let your husband treat you badly, your daughter will learn that having a revolving door and being treated badly by her husband is normal and acceptable. It is not. Your son will learn that he can come and go in his future marriage however he chooses and that his wife can be treated poorly and it is normal and okay. It is not.
You have set up a routine with the kids and I bet they are thriving. If you allow your husband to come in and disrupt it at his whim, you are teaching the kids that he is superior to you. He is not.
You have already proven to yourself that you do not "need" him. Look at what you have done! You have a job AND you are going back to school. WOW! Look at you go. You are succeeding with your life and the lives of your kids. Do NOT feel bad about that and do NOT let your husband negate that. He is out partying and God knows what else. Yes, his priorities are screwed up. Yours are not. You do not really, in all honesty, know exactly what he is doing.
Let me tell you about a friend of mine. She was not married, but was in a committed relationship with a man. Fortunately there were no children involved, but he went through that revolving door and she let him back in a few times too many. She is now at the end-stage of AIDS. I have sat here and watched one of my dear friends waste away because she too let her partner use a revolving door. Her mother has terminal cancer and less than a year to live. She will probably not outlive her mother and she is just 45 years old. Her death will probably cause her mother to lose all will to live. Please, shut the revolving door before you get caught in that mess as well. I never thought I would know anyone who had been affected by AIDS. I was so wrong. I have the one friend with full blown AIDS and two other friends (who have kids) with HIV. All because of unfaithfulness. It is too scary to run the risk.
If your husband feels that his partying is more important than his daughter (or his marriage) jsut let him go. You and the kids will be much better off with out him. I know it is hard to see that right now, but you will. My 10 year old son's father has not seen him since he was 2 months old. I don't even know where the creep is. I do not collect any child support because he has always managed to evade being served. I sell Avon for a living and am doing fine. I have never bad-mouthed my son's father to him. When my son started asking questions, I told him the truth in a matter of fact unpartial as possible way. He has not asked about him in at least 4 years.
You CAN do it on your own. You are already doing it. I commend you for your efforts and sincerely hope that you continue the way that you have been. You are teaching your children to not dwell on the past and it's hardships, but to turn the lemons into lemonade. BRAVO MOM!!!
Okay, now I am going to go read the other responses you received. Again, if this came across as harsh, I apologize. That was not my intent.
Good luck to you.
Oaky, now that I have read the other responses, I do have more to add. I agree that you need to go to an attorney. Even if you are not ready to file for divorce, you still need to have the custody handled. You need to make sure that the court has record of you having physical custody of yall's daughter. The other mom's are right. Without it being recorded with the court, he could come and get yall's daughter at anytime and disappear with her and not be breaking any laws. Now, it is my understanding that after a period of time (which will seem like eons) then charges can be filed against him, but they will likely be dropped before it ever makes it to court because there are no custody orders on record. The attorney general's office's main concern is to get child support for the child. However, they will set up visitation as well. When the parents do not or cannot agree on visitation, it is usually (based on the experience of a few friends and neighbors of mine) that the visitation clause states "as agreed upon by both parties." It is pretty broad, but it gives you the leverage to say, "No, you cannot come see her at 8:00, because she will be getting into bed. You need to come at 6:00 or make it another day when you can come earlier and not interfere with her routine." Also, in the state of Texas, if your husband were to show up at daycare (and later school) with a copy of yall's daughter's birth certificate and his driver's license and he can walk out of there with no questions asked. He cannot do that if you have a court order and provide a copy to the daycare.
Okay, I think I am done here. Oh! I do wish you the best of luck. I think you have done FABULOUS so far.