Life is less bearable and colourful without her but she's not mine

It's taken a lot to get me to this point, but here i am. So I've been friends with Mrs.S for about 2 years now and we have grown quite close as friends to the point where we see each other up to 4/5 times a week but the only problem is that getting to know her meant realizing that she's the closest thing to a soul mate i have ever come across. We're also in that gray area where we talk about most things but don't really talk about possible future relationships or try to set each other up with people and stuff. That might sound like your average teenage worry but there have recently been these moments where I'm holding her like she's my everything or telling her she's amazing and she just doesn't understand... I keep acting like everything is fine and we're just friends, sleeping over at her place is a common occurrence too but I can't bring myself to speak up because as the title says, if i ruin it and loose her life is going to get a lot more bland and grey. And now for the cherry on top she is also the ex of one of my close friends and they broke up 3.5 months ago, and it hurts so much to see her still long for him when he's already seeing a new girl. I just want to hold her and keep her safe and make sure that she knows what an incredible person she is and that i would always be there for her but I'm also terrified of destroying the relationship with both of them(obviously her more than him) in the process.

We do not have sex no, personally I'm the type of person that prefers being in some form of a relationship before moving forward with more serious commitments and possibly complications by having sex. I have deliberated greatly and decided to ask her out to lunch later this week and she has agreed, we will be going to a Mexican place she used to visit when she was younger, i figure its a safe space for her and a good chance for me to get to know more about her. Unfortunately I have been avoiding relationships for some time now and I'm not entirely sure how to bring up the topic of my romantic emotions for her without being too serious or reminding her of her past relationships. Thank you for the advice and i hope you will assist me further as i really don't want to mess this up.