I've had the same problem.
Gratitude implies a dependent relationship between yourself and another, or yourself and life, which is why you resist/resent it. When you have little to feel thankful for, expressing gratitude can have the contrary effect of humiliating you when you already feel low.
Instead of gratitude, think in terms of appreciation. Appreciating what you do have, and the help that you have received, does not put you in a dependent relationship.
Appreciation is a feeling that can be expressed between equals, without the implication of debt. "I appreciate what you've done for me" is very different in connotation from "I'm grateful to you", but it achieves the same end (acknowledging their effort). You can appreciate, for example, the fact that someone is trying to help you, but you might resent expressing gratitude if they haven't actually helped you. Gratitude is more properly reserved for situations in which you have asked for help and someone has given it to you. (If you have difficulty expressing gratitude in these situations your problem is more serious.)
It's hard to feel grateful for your life if you're suffering, but it's not that hard to appreciate the good things that you do have. I do not feel grateful to have the life that I have (eg. physical pain), but I have no problem appreciating the good things I do have (eg. the movie that distracts me from that pain). A homeless person may find it hard to feel grateful for the life that they have, but they can appreciate fine weather when the sun comes out. Appreciation is about directing your attention away from your negative experiences to whatever positive experiences are available to you.
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