I think you already know why you are still insecure about “mark”. At 21 years old, the experiences you went through with both men is still recent and unresolved. Mark left you a number of times for someone else. Even though your rational self reminds you that you were both teens at the time, your not-so-rational and emotional self is still not able to trust him 100%. Rick was trustworthy but didn’t make your heart sing enough. Mark made your heart sing but you still don’t entirely trust him. Of course you are confused.
To add to the mix: Your mother has taken the side of Rick. You say she “hates” Mark. Mothers are generally protective. It’s possible she sees things in Mark that concern her but that you aren’t able to see. Or not. Maybe she just likes Rick more. I can’t tell on the basis of such a short report. But it might help you if you asked your mother (calmly, not defensively) what it is that bothers her about Mark as a potential son-in-law.
One way out of your spin is to ask yourself what you think needs to happen for you to forgive Mark enough that you can totally trust him. Do understand that there is probably nothing he can do at this point that he hasn’t already done. Trust is a gift we give someone when we think they deserve it. It’s up to you to decide. Be honest with yourself. Is your insecurity about him at all justified? Is it all in your own head? Or are there things you still need to talk about with him for you to be with him 100%?
This is important work for you to do if you and Mark are to last. Love is not enough. A relationship that lasts is one where there is total commitment and trust.
I wish you well.