I have a friend that's in a relationship but I can't help but to have feelings for them.
Why are you having romantic feelings towards them?
I love having the feeling in my head when I'm with him.
it is risky to have feelings for someone that is not available. This could cause you to interfere in their relationship which is wrong.
You are so pessimistic. I'm going to continue this forever.
I highly recommend you don't do this, there is a high probability this could destroy your friendship. I would focus your mind elsewhere and try to find someone that is currently single.