It's counter-intuitive but maybe you're not aroused enough. Imagine your cock getting really hard
D), find something to focus on that's more arousing than you're used to, and go much slower as Juliana mentioned. If you can carefully build up for 15 slow minutes, yer buddy might "suddenly" break through to a new stage -- and get harder than you ever imagined it could, and maybe you'll last 30 minutes in this huge, steal-like state. Think "prolong this pleasure state. Cum and it's all over. Slow down. Stop." And once you're on the edge, try to gently squeeze your penis head for a minute and start over slowly.
I had my first tantric with Maylee today and I was amazed at how long I lasted. There's your trainer.