Is it obligatory for a girl who had a past in relationships and then repented from it to inform a suitor about it, or is it better for her not to marry for fear of her past being exposed in the future?

As long as you are repentant to Allah, rejoice in the acceptance of your repentance. Strive in obedience to Him and do not pay attention to the whispers (waswas). Repentance erases the effect of sin, and one who repents from sin is like one who has no sin. Do not fear marriage and do not turn away from it. If a righteous man proposes to you, accept him. Be careful not to inform him of the forbidden acts you committed, for it is obligatory to conceal one's own faults. And do not fear disgrace or punishment as long as you are repentant, for the repentant will not be punished neither in this world nor in the Hereafter. So rejoice in good, think well of Allah, and supplicate to Him frequently.