In a fair bit of trouble and seem to be doing alone

[Anocxu]

well, basically became best friends.

If you met this guy >after< your partner. .
How come your partner was not your best friend?

This new friend was single, whilst I was in a relationship and everything seems normal, tried my best to ensure he never felt like a third wheel or to throw things in his face so >I kept things relatively separate.<

^^^^
This is where you made a terrible mistake.
If you had presented you and your partner "consistently"
As one.. as a team.. it would have helped to keep your feelings in check.

You should have set aside time spent with you , your partner and your best friend.

Example the three of you for dinner , a movie.. a game .. minimum twice a week.

The reason you kept things separate ..you were attracted to him way before you realized it.

>You and your partner are one..
>You present yourself as one..
I'm sure you two separate from time to time.. but two much separation is just an awful way for you to grow apart.

I spent a lot of time with my friend because the other half really likes to go and do his own things.

^^^
Personal growth is essential for a successful relationship. .
BUT..
Your partner going off and doing his own things should have been an indication to you to get involved in His space just a bit more.

The issue is, my partner has no clue, he has no clue, and I have become very distant and distracted over this all, and I’m seeking to resolve this.

There is definitely a possibility your partner knows.

Be careful with that assumption.

You haven't said much about your partner..
You are way too invested in your feelings for your best friend.

How about this..
Come clean with your partner..
Both of you can work together to get your feelings in check?

I was in a similar situation. .
It didn't go very well..

I realized what my partner was doing waaaay before he did.