Passwords and security issues

[PTCRUISER]

Dear friends, first of all allow me to say that I am not an experienced user.

1. I have my gmail with a user name and a password.
2. My gmail has been connected to the outlook so for me not to open every time the internet browser.
3. I tried yesterday to enter gmail through internet and I was not allowed. Most probably I changed the password and I don;t remember - keep in mind that I continue to send & receive though outlook.
4. In the past I remember I gave an alternative e-mail address (just to prevent such cases), but I gave an email from the company I was working (actually I am not working there, and I don't have access to this e-mail).

Questions
a. Please advice me what to do
b. I suspect that someone else has access to my current emails (is this possible?) Can I check it? Can I avoid.
c. I don't want to change my user name (is very important for me)

Thanks in advance for your help