Loving someone with mental health illnesses is not easy. In fact it is the hardest thing that you will ever do. The person in the relationship that has mental health illnesses may not always be able to express what's wrong when they are having a bad mental health day.
The best thing for you to do when your partner is having a bad mental health day isn't to continually ask them what's wrong or what you can do to help. The absolute best thing that you can do for your partner is to just wait until they can talk to you.
Continually asking them what's wrong or what you can do when they can't yet tell you what's wrong is just going to make your partner more frustrated in the fact that they can't tell you what's wrong. Sit with them and be there for them and let them talk when they're ready and able to talk to you.
Some people with mental illnesses already think that they are hard to love or that they are never going to find real love. If they feel like this it's most likely because any past partners they have had haven't understood the mental illness or illnesses that they have. It takes time to really understand a mental illness and understand how it affects your partner. Don't give up on your partner before you understand the mental illness that they have and how it affects them.
Even if you have had a partner that has had a mental illness doesn't mean that the same mental illness is going to affect a future partner the same way that it affected your past partner.
Let them know that you will be around for them. If you don't understand their mental illness be up front and honest with them and tell your partner that you don't understand so that maybe they can help you to understand. Don't push them to talk before they are ready and once they are ready they will talk to you.