Would you take her back?

My (33M) SO (29F) went on a summer trip with friends, was supposed to return after a week, but eventually called me for the airport to breakup; I was supposed to pick her up once she landed.

Turns out, she slept with a male friend of her friends she’d travelled with for about a week.

Came back, begged for forgiveness, I succumbed, but I just can’t stop thinking about it.

I feel like I’m only staying in this because I’m afraid I’ll be left alone, at this age. We’d been together for two years or so.

Thanks boys.

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Edit: this is truly heartwarming, I just landed and saw all your responses..what can I say.

Thank you, boys, I truly do.

Can’t wait to read your insights. Wow.

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UPDATE: I left her. Growing a spine hurts like hell. But, I left her.

Thank you all, boys and girls, these few days, your messages kept me going. What seemed impossible, happened..

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UPDATE II: I'm still gone, despite the calls, the tears, the yells, etc. Also, my cat - which she had asked me to get rid of in the past - seems quite proud of me these days; or it's just me.