The Most Important Lesson You'll Ever Learn (or Fail to Learn)
Romantic relationships are not supposed to feel exhausting. The time you spend with a romantic partner is your free time; the things you do with a romantic partner are supposed to feel like things you enjoy doing during your free time. The time you spend with a romantic partner is not supposed to feel like an unpaid second job that you force yourself through the motions of because you feel obliged to. If it starts to feel that way, then you're not getting what you need from the relationship. If it's felt that way often for a significant portion of the relationship's duration, you really need to dump the person. If for most of the relationship neither you nor your partner felt that way at all, but just lately one of you has been starting to feel that way sometimes, then something is still very significantly wrong and it would probably be a good idea to seek relationship counseling.
It's extremely unlikely that you'll ever create a good relationship by forcing yourself to remain in a bad one and desperately hoping that it'll turn into a good one if you just stick it out long enough. Bad relationships tend overwhelmingly to get worse, not better, with the passage of time. The best way to create a good relationship for yourself is to ditch the bad relationships and learn to enjoy the single life while holding out for someone truly wonderful, who you'll consistently look forward to spending time with instead of dreading it.
(This journal entry was inspired by watching too many LiveJournal users feeling obliged to remain in relationships that are obviously making them unhappy.)