Along the way from the pilot episode to Ted actually meeting the titular 'mother', the How I Met Your Mother gang had their fair share of heartbreak - even Lily and Marshall, who are in a solid relationship from the start. And while many of these were fun relationships that had to end, or relationships that fans didn't particularly like in the first place, a few of them were absolutely heartbreaking to watch.
The characters are human, after all, and sometimes they made huge mistakes, put themselves out there and got shot down, or just realized that love wasn't enough (and sometimes, chemistry and timing isn't enough either). From Ted being left at the altar to the breakup that made Barney Barney, these are the hardest hitting splits of the series.
Barney & Nora
It's not so much the end of Barney and Nora's relationship that makes this one so tough, but the circumstances - and the head shake. Barney and Nora actually had a decent relationship, and it would have been nice to see it continue, but fans were rooting for Robin and Barney at this point in the show - and Barney broke up with Nora after he and Robin sleep together, and decide that they should give it another try. However, while Barney calls it quits with Nora, Robin stays with Kevin, choosing not to tell him that she cheated. Watching Barney realize this as she shows up to the bar with Kevin, and then seeing him clear rose petals off her bed, is absolutely devastating.
Barney & Shannon
This breakup happened long before the show begins, but is slowly revealed in flashbacks, and sheds some serious light on Barney's history. Far from the womanizer he becomes, as a young man, Barney was saving himself for marriage, and wanting to go save the world with the woman he loved... but she ditched her hippy ways, dumped him, became a corporate type, and broke his heart. His powerful, besuited persona is a reaction to this breakup, and fans can't help but feel for the younger, more naive Barney, and the first time his heart got stomped on.
Robin & Don
Robin doesn't always find it easy to forge relationships and really commit, always choosing to put her career and her own desires first. That's why the end of her relationship with Don was so painful - she actually decided to put a relationship first for a change, and it blew up in her face.
She turned down a dream job to stay with Don, but he ended up being offered the job next, and taking it - leaving her without the relationship or the job, and deeply regretting prioritizing the romance over work. She didn't deal with the breakup well, either, and it took her a long time to get past this one.
Barney & Quinn
Barney, like Robin, struggled with the idea of commitment, but when he and Quinn got together, they seemed like a perfect match. They were so perfect, in fact, that Barney actually proposed - and they very nearly got married. In the end, though, they broke things off with the realization that they didn't actually trust each other. Both wanted extremely detailed prenups, covering every possible scenario, and thankfully, they realized that was a red flag. It may have been the right decision, but it was still heartbreaking to see.
Ted & Victoria
Victoria and Ted actually break up twice in the series, and both times it's a gut-wrench. The first time they break up it's partially because they struggled to make a long-distance relationship work, and partially because of Robin (and the fact that Ted cheats on Victoria with Robin). This was painful enough, especially as Robin then finds out that Ted hadn't broken up with Victoria before sleeping with her, and they don't actually get together because of it.
The second breakup, however, is even worse - as Victoria realizes that although she and Ted are great together, he and Robin are just too close, and she can't deal with it. Ted can't cut Robin out of his life, and it means that Victoria is the one to leave.
Robin & Kevin
Although Robin and Kevin stay together despite the fact that she cheated on him with Barney, they do eventually break up - and it's for a deeply realistic, and incredibly painful reason. They are absolutely fantastic together, but after Kevin proposes, it becomes clear that they don't want the same things. Kevin knows that Robin can't have kids, and is enthusiastic about potentially adopting... what he didn't realize is that it's not just that Robin cannot physically get pregnant, but that she doesn't want children at all. Seeing them come to the realization that a great relationship just doesn't have a workable future was incredibly sad - even if it was the right ending.
Robin & Barney
Robin and Barney have more than one breakup in the series, but it's the final one that really hurts. When their relationship first comes to an end, it's a mutual decision that makes perfect sense - they were bringing out the worst in each other, and they were both unhappy.
That ending is sad, but not particularly painful. When they decide that they have to get divorced (in the flashfowards), though, that was absolutely devastating for fans. In the end, the show decided that Robin's constant travel for work meant that the marriage just didn't work, and that neither of them was happy... and neither were fans, who thought that this was not just heartbreaking, but unrealistic, and an unfair ending for a great couple!
Barney & Ted
Barney and Ted clearly didn't have a traditional 'romance', but when their 'bromance' included a breakup, it was surprisingly tear-jerking. After Ted and Robin broke up, Barney and Robin slept together, and for Ted - with his 'one true love' obsession and feelings for Robin - this was a dealbreaker in their relationship. The two broke up after Barney told him about it (and after Ted punched him), and only reconciled after Barney was hit by a bus.
Ted & Stella
Ted may not be every fans' favorite character, but even he didn't deserve the blow that Stella dealt him when she left him at the altar for her ex. Ted and Stella had a whirlwind romance, and even if not everyone felt that they should have been getting married so fast, Ted was convinced that he had found the one, and that he would live happily ever after with her (in New Jersey). Watching him find the letter that she left, and seeing Robin notice Stella and Tony together on the ferry, was gut-wrenching.
Marshall & Lily
For almost the entirety of How I Met Your Mother, and long before that, Marshall and Lily are the perfect couple. They may have their disagreements, but they are incredible partners and incredible parents, and were fan-favorites for being so wholesome and good together. So when Lily left Marshall to try and pursue an art career, and Ted came outside to see Marshall sitting in the rain, holding her engagement right, it was truly a devastating scene.