Hi Tati & welcome.
Some people say that you can only love one person at a time. Some might say that, if you can't choose, then you can't really be serious about either of them, and your Mr Right is elsewhere.
I'm not sure. I do think it is possible to love two people at the same time, but you can't really be in a successful relationship with both.
Some try ~ married people have affairs, after all.
Is it possible that you get different things from each boy?
You stayed with your boyfriend, even when he was unkind to you.
You only considered dating your best friend, after you found out that he was in love with you.
Is it possible that you love your boyfriend more than he loves you; and that your best friend loves you, more than you love him?
Presumably you are very fond of your best friend?
And he is good to you.
So this relationship must feel really warm & pleasant, if you are used to a boyfriend who isn't so good to you.
On the other hand, you have stayed with that boy for a pretty long time, considering he wasn't good to you, and that must have left you feeling empty at times.
Be honest with yourself, so that you can be honest with them. Is it the way they treat you that makes you confused, or the way you feel about them?
Is the best friend giving you the warmth, love, security & attention that you would like to receive from the boy you love?
Are you actually in love with either or both of them?
The issue with your boyfriend is separate to a degree. If he is treating you badly, and you are already dating someone else, then this does not sound good for you and, regardless of what happens with your 'best friend', you should really consider what is right for you.
It's only fair on the boys, as well.
Two-timing isn't usually a good idea.
Maybe a break from both, for a while, would give you time to consider, while you are not under pressure?
I don't know how old you are, but, especially if you are quite young, this can have a bearing on the situation.
Good luck!