I feel like I might have committed shirk purposely and knowingly after I accepted Islam. Do you think Allah can forgive? Ever since I found out, I can't move on and my mental health is in bad condition. I want Allah's grace and forgiveness but I can forgive myself, and I feel so bad.
Do I still have a chance to enter Jannah?
No one here is qualified to tell you whether you have a chance to enter Jannah or not, this is absolutely on the hands of Allah (SWT). But what we can tell you is, no matter how much sins you have committed and how terrible you feel, do not despair of the Mercy of Allah (SWT). Allah (SWT) will forgive anything if you sincerely repent to Him.
Say to those who have disbelieved [that] if they cease, what has previously occurred will be forgiven for them Qur'an 8:38
Say, "O My servants who have transgressed against themselves [by sinning], do not despair of the mercy of Allah . Indeed, Allah forgives all sins. Indeed, it is He who is the Forgiving, the Merciful." Qur'an 39:53
In regards to committing shirk, even though you did not specify what type of shirk you've done in the past, always seek refuge in Allah (SWT) from all types of shirk. The Prophet (PBUH) taught us this Du'aa:
اللهم إني أعوذ بك أن أشرك بك وأنا أعلم وأستغفرك لما لا أعلم
O Allaah, I seek refuge in You from associating anything with You knowingly, and I seek Your forgiveness for that of which I am unaware of.
Allah Knows best.
