When Harry asks Dumbledore why Merope, as a witch, didn't use magic to help herself after her husband abandoned her, Dumbledore replies:
'But it is my belief - I am guessing again, but I am sure I am right - that when her husband abandoned her, Merope stopped using magic. I do not think that she wanted to be a witch any longer. Of course, it is also possible that her unrequited love and the attendant despair sapped her of her powers; that can happen.'
HBP, British paperback edition, pages 310-311
Dumbledore goes on to reference Merope's 'long suffering,', but she is portrayed as not being able to use her powers or deciding not to use her powers and giving up her will to live because of her husband's abandonment. For most other people, I would find this melodramatic and also sexist, considering this is a series in which two women and no men have stopped using their powers (for whatever reason) due to unrequited love. With Merope, however, I don't believe that Tom Riddle senior leaving her was the main reason for her despair. I think it was the last straw in a series of long misfortunes, and that Mrs Cole's brief description of Tom Marvolo Riddle's birth reveals a lot about Merope's personality and state of mind.
It's hard to imagine a witch being born into worse circumstances than Merope. Her very genetics are against her, due to the Gaunt family's history of inbreeding; her eyes are crossed, and though it is hard to determine her intelligence, her brother Morfin is mentally disturbed and very possibly mentally retarded. The Gaunt family is impoverished, living in a filthy shack in the woods. As the Chamber of Secrets could not open until Salazar Slytherin's heir returned to the school, it appears that Merope never attended Hogwarts. (Her father Marvolo's comment that he doesn't open letters also supports this theory.)
When it comes to family dynamics, Merope has no ally. Both her father and her brother insult her and put her in charge of the menial tasks around the house. Her mother is never spoken of, and either has died or left the family. The Gaunts, surrounded by filth and wary of strangers and Muggles, appear to have no friends whatsoever. Emotionally, Merope is alone in the world. Harry's first thought upon seeing her is that "he had never seen a more defeated-looking person" (244).
When Marvolo and Morfin are taken off to Azkaban, Merope has suffered eighteen years of emotional abuse. I think very few people would blame her if she stayed at home waiting until her family returned, afraid of the outside world. However, Merope, the most defeated-looking person Harry has ever seen, is not defeated yet. Through whatever means, she manages to elope with Tom Riddle senior and leave behind her life of squalor.
Tom senior is a very understandable target for Merope's affections. He is everything her father and brother are not; rich, good-looking, living a life of privilege, and, if his behavior towards Cecilia is typical, protective towards women he cares about. ('Don't look at it, Cecilia, darling' (249).) Living on the other side of the valley, he is physically close enough for a relationship between them to be possible. Tom is like a rescuer from a fairy tale, complete with horse.
Given Tom's repulsion by the Gaunt house, Dumbledore is probably correct in assuming that Merope used some magical means to convince him to marry her. This puts Merope's method of escape into questionable moral territory. A love potion, used in the way Merope uses it, to make a man marry her and conceive a child with her, seems to be the magical equivelent of a date rape drug. This would seem to cast Merope into the category of despicable without question, but it appears Merope has never been raised with any sort of moral standard. Isolated from the world outside her dirty cottage by her malicious father and brother, it is surprising that Merope had the inner strength to move past the prejudice towards Muggles she had been raised with enough to want to marry Tom. Exposed to no love that we know of, it isn't surprising that Merope believes it can be artificially created.
Unless Merope was suddenly no longer magically able to keep Tom under her spell, or unless Tom in his enchanted state somehow broke through it, Dumbledore is probably also correct in assuming that Merope voluntarily stopped using magic over Tom. If she believed he would love her anyway, without magic, her character is naive and hopeful. If she realized that using magic to make Tom love her was wrong, her decision to stop is enormously brave. For Merope to realize that the error of her ways and to cease giving Tom the potion, unsure if he would continue to stay with her, is for her to risk her future and happiness by doing the right thing. Either way, naive or brave, Merope is not malicious.
If Merope's powers were lost due to emotional upheaval, I don't believe it was solely the result of Tom leaving her, or perhaps even mainly because of Tom leaving her. Tom's abandonment, I think, would have been the final thing to tip her over the edge into full-fledged despair. Merope suffered eighteen years of poverty, no formal education, verbal abuse, and a generally loveless life. For Tom's abandonment to be the reason why she lost her powers minimizes the terribleness of her previous life. It makes her into a lovelorn Echo pining over Narcissus and wasting into nothing, and I think Merope's ability to keep going for such a long time in the face of horrendous adversity invalidates that interpretation.
For whatever reason, Merope stopped using magic. Yet though she seems to have given up on her own life, she has not given up on the life of her unborn son, as Mrs Cole's story reveals.
'And this girl, not much older than I was myself at the time, came staggering up the front steps [of the orphanage].' (315)
Though Merope will be dead within another two hours and is staggering as she walks, she goes to a place where she knows her child will be taken care of.
'I remember she said to me, "I hope he looks like his papa"' (315)
Merope wants her son to be in the mold of her past beacon of hope, even though she has no hope left.
' - and then she told me he was to be named Tom, for his father, and Marvolo, for her father - yes, I know, funny name, isn't it?' (316)
Merope's feelings towards her father cannot be completely negative if she wants her son to share his name. Overall, Merope seems to be emotionally confused, with loyalties in different places. She wants her son to be named Tom even though Tom has left her; she wants his middle name to be Marvolo even though Marvolo called her a pointless lump and a dirty Squib. She emerges as a poignant figure, attached to those who do not love her and alone in the world.
Merope could have turned out to be a horrible person. She could have been a female Morfin, assaulting Muggles and nailing snakes to the door. Or she could have given up much sooner than she did, living a life of constant misery with her family, their slave forever. But she tried to find happiness for herself, though she was limited by her lack of experience with the world and her lack of moral upbringing. Even when she had nothing left, she still wanted something better for her son. Merope gives up on herself, but she never gives up on her idea of having a family. As Tom is an odd parallel to Harry, Merope is an odd parallel to Lily, still concerned for her son even when it's apparent that she herself is going to die. I wouldn't want to be her, but I admire her fortitude and her hope for something better up until the moment she dies.