You get to feel however you feel about this.
You aren’t obligated to feel differently. Period.
You aren’t obligated to be fair to him or give him another chance or anything else. You’re allowed to break up with a boyfriend for any reason at all. This is a fairly compelling one, all things considered.
You aren’t obligated to marry him no matter WHAT he does to try to convince you. You don’t have to have a "good enough" reason not to marry someone you don’t want to marry. If you don’t feel good about it and aren’t sure you want to marry him, don’t accept a ring. Don’t marry him. Period.
Marriage is a long and challenging road dealing with all of the many things that life may throw at both of you: money stress, job change/loss, illness and aging of loved ones, illness and aging for both of you, CHILDREN, potential challenges for your children, and SO much more. And yes, it also includes potential temptations along the way. Don’t marry someone that you’re not completely convinced is in love with YOU (as a person, not as a concept or a convenience or a benefit) and committed to going through all of that with you and only you by his side.
You don’t have to be with him just because he wants you to. You can see that he is exploring options in addition to you, that he is thinking about both you and her. This may not technically be infidelity, but it is DEFINITELY a red flag on his emotional health and stability and it is DEFINITELY a red flag that he likely isn’t ready to make the kind of emotional, physical, financial, permanent commitment that marriage is.
Your gut is telling you something. LISTEN to it.
If you don’t feel like you want to marry this person, DON’T. You don’t need a reason or an excuse or a pretext or anything else. You get to marry who you feel is everything that you want in a partner. Full stop. You also get to decide to never marry if that is what suits you. The important thing is YOU get to decide YOUR future and who you spend it with.
You are young. You don’t have to commit to anything with this person. There are plenty of good men out there.
Listen to yourself and do what is best for you for the long term, not just to make anyone happy at this moment with a ring and a wedding and a party. He can pursue his mental health issues and down the road hopefully be a better partner for someone, but that is about his path.
Your path and your happiness and well-being should be your only concern at this stage of your life. Many of us here can attest to the fact that it is never a good idea to put someone else’s wants, needs and goals ahead of your own—particularly when you are single and have the choice to take any road. Later, life can get complicated by many other things. Right now, the choice is only about you and your choices for your life.
Trust yourself and do what’s right for you.