Asalaamu alaykum,
A learned person with more knowledge than me might be able to be of more help. However, this is from what I understand and have searched on islamqa. May Allah (swt) forgive me if I have made any mistakes is advising you and may He make it easy for you. Ameen.
First of all, you say you THINK you might have committed shirk. Perhaps it might help you to ask an imam or a mufti about this to make sure. From what I understand, shirk can only be committed if you fully intend it from the heart as well. The following link has some useful information about the different types of shirk we might commit and perhaps it might help you to identify if you did commit shirk.
http://islamqa.com/en/ref/34817/repent shirk
As regards to repentence, what you need to remember is Allah (swt) is so Merciful and as long as you repent sincerely from your heart, insha Allah you will be forgiven.
I'm not sure if I can post links in messages but since I believe these will be helpful to you, I will post them here. Apologies to mods if I'm not meant to be doing so.
The following links are both from islamqa and about people who have committed shirk and wants to repent:
http://islamqa.com/en/ref/34171/repent shirk
This one is about HOW we can repent from shirk:
http://islamqa.com/en/ref/20482/repent shirk
With regards to a ghusl, you might need to clarify that with someone else. I am not the best one to advise you on that since I always think I have committed kufr due to bad waswas I get and feel like I need to do ghusl. Hopefully another member with more knowledge than me will be able to guide you here.
What I would say is that, even for the biggest sins like shirk, Allah (swt) accepts our repentence because he is Merciful. Also, if possible, try to talk to an imam or someone about your particular problem and ask them whether it is shirk since you seem unsure if it is or not.
May Allah forgive you and make things easy for you. Ameen.