Lendl's easiest run to the Wimbledon semi finals was in 1983, ironically. Lendl then lost a close 3-set match to McEnroe in the semi finals, with underdog Chris Lewis waiting in the final. I think this is what sowed the seeds for Lendl in convincing him that he could win Wimbledon. As each year passed, his obsession with winning Wimbledon grew, reaching its peak from 1989-1992. He gave everything, and it wasn't enough for him to win it.
Regarding his Wimbledon finals of 1986 and 1987, Lendl had missed opportunities. In 1986, Becker was 0-40 down on serve late in the third set, giving Lendl set points. In 1987, while Cash was in peak form in the second set, Lendl led 4-1 and 5-2 against Cash in the third set. On both occasions, Becker and Cash got out of the holes and won the third sets, and won the match with it.
The 1989 semi final against Becker was the most painful for Lendl though. Lendl was as close to tears as you'll see him at the end, shaking his head in frustration. Lendl had levelled the match at 1-1 in sets and then went two breaks up in the third set with huge momentum before there was a rain delay. When the match started again, the momentum was more in the balance, but Lendl won the third set and was briefly a break up in the fourth set (at 3-2, I think). Becker got out of that hole and won in 5 sets, and Lendl had a few frustrating line calls go against him too.
1990 would have been incredibly frustrating for him too, because he played so amazingly good grass-court tennis at Queen's Club, and then failed to replicate his amazing Queen's Club form at Wimbledon. He was outplayed by Edberg in the semis at Wimbledon that year.