I understand how my gf comes and how to give her an orgasm, but I've reached a point where I'm literally too tired to have sex

My girlfriend sleeps with me sometimes (we live in different cities so it's often, but not every night). And we always have sex before we go to sleep. But, honestly, I'm just so tired. When we sleep together, I'm working a 14 hour day and I still have to make sure I can go down on her or whatever because she wants that kind of tending. <br><br>I'm not complaining, but she would get mad if I don't. I'm afraid one night I won't be able to and she'll get upset. I've talked with her before about how I'm tired and she has acknowledged that, but I know with her track record that she would be upset about it. <br><br>I don't know what to do. I want to make sure she has an orgasm, but I literally cannot function at the end of the night.