That's a hard situation for all involved.
My parents separated when I was about 4, so I had many years of experience dealing with being my mother's primary companion at home, and the weekend visits with my father. Your situation is a bit different, though, as my parents had had a lot more time to adjust by the time I had reached a more independent status.
First and foremost, NO, this is not your fault. This is the message EVERY parent going through a divorce or separation should absolutely be emphasizing to their children constantly. Unfortunately, I think that adult children sometimes get overlooked for stuff like this, even though it absolutely helps to hear it.
Putting aside your emotions and doing what you please is self-contradictory. The conflict exists because you want to do both. Ideally, you find a way to balance both sides of the equation, but sometimes you wind up in a position where you have to choose one or the other (sometimes you can make this a temporary state of affairs, and restore balance once the situation stabilizes, but not always). If it gets to that point, you'll have to do some serious thought and make some tough choices, but there are some things you can do to help reduce the chances of being forced into such a decision.
A good place to start might be for you to decide what your "ideal" situation is. If you could eliminate all feelings of guilt or obligation from the equation, how much time do you honestly want to spend with each of them? This can be very specific, or a flexible, general range, depending on what you want. Get an idea of this, and tuck it away in the back of your mind. Don't think of this as a goal, though, but rather a measuring stick to see how close to your ideal balance you wind up at. Most compromises don't wind up with either side getting exactly what they want, but knowing what you want helps keep you focused on getting at least somewhere in the ballpark.
I'd recommend that you talk to your parents. "Confront" is probably too harsh of a term, but you definitely want to open dialog. You'll probably get better results if you start out by asking questions in an attempt to better understand your parents' perspectives, and then introduce your perspective in such a way as to try to work for a solution that addresses everyone's wishes.
Ask your mother if she feels she has too much time to herself, and what she wants/expects/hopes for from you. It may be that your guilt about leaving her alone is unfounded, and maybe she's simply enjoying relaxing, or is taking the time to think, or just really wants to catch up on television. Ask her how she'd prefer to spend her time, both during the day and during the evenings. Tell her that you feel bad about coming home late and missing seeing her, but that you also don't want to feel like you're neglecting time with your friends.
Tell your father something very similar. Tell him that you want to spend time with him, but that there are other things that you also want to do on the weekends, and sometimes you feel frustrated by the conflict.
In both cases, you may be able to work out a schedule that works for both of you. See if you can set aside blocks of time on specific days that work for both of your schedules, and dedicate that time to a shared activity. Perhaps Saturday afternoons become "lunch and a movie with Dad", or Wednesday nights become "take mom out dinner night" (or "make dinner together with mom night").
If, after talking with them, you find that you're not able to come up with a solution that makes everyone happy, you may want to consider speaking with a therapist or social worker. A neutral third party can be very helpful in situations like this, and a professional therapist or social worker can be an excellent listener, and can help suggest possible compromises or methods of coping with your frustration. Many employee health care packages include some sort of mental health services, so you may wind up being covered.
As an alternative, members of the religious community such as priests, pastors, rabbis, etc. are good resources, and many take regular training on counseling techniques.
Through this all, you should also keep in mind that the situation may change. I'm not saying your parents will get back together (although it has been known to happen), but your relationship with your parents likely will evolve over time.
If they stay separated, they will want to start rebuilding their lives, and may find other people that they want to spend time with. That doesn't mean they won't want to spend time with you, too, but it will alleviate a large part of the burden of guilt you might feel, knowing that they have other people to keep them company.
Also make sure that you don't stop living your life, just to fill a void in your parents' lives. You need to have opportunities to advance your own relationships and career, and there are extremely few situations where putting those completely on hold justifies the damage to your long-term happiness, even if only temporarily.
Best of luck to you!
