Honesty is
ALWAYS better.
You can't hide you mental illness forever, as it impacts your daily life. You will eventually have a rough period and it's going to be hard to make up excuses so it seems like you don't have mental illness. When people care for you, they care for YOU. All of you, and that includes your mental illness. It can be scary to share that with someone, but in the end you'll feel better than you will if you hide it and, inevitably, sabotage yet another relationship.
Allow me to share a bit about my own life:
(triggering: SH)
I was diagnosed with mental illness at the age of nine. I've been hospitalized many, many times for my psychiatric condition. I started self-harming when I was fifteen to cope. My scars cover most of my body. So for me, hiding my mental illness was never an option. I tried, but it didn't work out, and the scars made it obvious.
I was scared of what my partners would think, if they would judge me. I'd warn people before meeting that I had a lot of scars. And you know what? Not a single one of them judged me for it. They were all understanding. With the people I eventually ended up in relationships with they were there for me when I had breakdowns. They supported me. They didn't see me as a burden, or too much to handle, because they cared about me, all of me, and they wanted to be there for me.
You might be surprised by how people react to you. Most people will want to help and support you. One of the best pieces of relationship advice I ever got was to view you and your partner as being on the same team. You face life together. You work through everything together, and you have each other's back. When you have that solid foundation of trust and reliability there's nothing you can't face.
So be truthful with him. You don't have to tell him everything all at once, but as time goes on and your intimacy grows I suggest opening up to him about your struggles. Chances are he'll be supportive and caring.
Good luck, and
PM me if you need anything else.