People often link ambition to earning capacity because quite often, those two things go hand in hand. However, you can be ambitious without chasing a high-earning job.
Ambitious is wanting a better life for yourself and your loved ones. The OP isn't just earning twice as much as her DP now, she is ambitious about wanting to create a home for them both. A better future.
Her DP doesn't seem to be thinking about buying a property, and in unilaterally deciding to take a low paid job, he doesn't see to be thinking about how much financial pressure that puts on her either. He is not showing signs that he is thinking about wanting a better life for them.
It is the attitude that is getting the OP down, and it's unsurprising she is comparing her DP to others in her life and wanting him lacking. She doesn't deserve all the horrible comments in this thread - she wants someone who, like her, wants to create a better life for their partner.
Ultimately, I don't think this is a one-time loss of confidence. I think the writing has been on the wall for a long time. You cannot change someone to make them feel more ambitious - it's a personality trait. They either have it or they don't. I don't think they are compatible, unfortunately.
Incidentally, in my line of work, it would not be better to take 'any job' following a redundancy. It would make more sense to live off the redundancy and wait out a role at the right level. This is not the case in all lines of work, but it should be noted that OP and her DP have worked together previously, so her insights into what is normal should be accepted. If she thinks he should have waited it out, she's probably right. Why do we have to assume she's wrong?