Since your partner is an European national you would need to apply under EEA/EU rules, not UK immigration rules. First for a EEA family permit to allow you entry into the UK and then again for a residence card once you are in the UK.
But under EEA rules:
If you and your partner are not married or in a civil partnership, you must be able to show that you are in a durable relationship with each other.You need to provide evidence that you lived together in a relationship akin to marriage at the same address for two year sprior to the application. Joint bank accounts help, but they aren't necessary. You can keep separate accounts, as long as the statements are sent to the same address. The same goes for other types of evidence, utility bills, etc.
From the EEA application checklist
Unmarried partners: If you are living with a partner who is not your spouse or civil partner, you must provide evidence that you are in a durable relationship with them. You should ideally demonstrate that you have been living together for at least 2 years. (Generally you can meet this requirement by providing joint bank or building society statements, joint tenancy agreements, council tax bills or evidence that you are both paying utility bills at the property where you reside.)
From what you have explained, I don't think you and your partner would meet this criteria. You say you maintained separate apartments. It doesn't matter that you spent most of your time in yours or his you would have needed to live in one, together.
You do have other options, including getting married or coming here as a student, which I’m sure other people here have more experience with.